Since my last post I have read more of this thread and learnt a few new things, such as there are some problems with HP versions of the 4080N and that there is a HP version of 0S35, namely 0C35.
I tried the 0C35 and ended up with the same results as the Sony firmware.

The Nero debug log output is (apologies for the length):
Windows XP 5.1
IA32
WinAspi: -
ahead WinASPI: File 'C:\Program Files\Nero\Nero 7\Core\Wnaspi32.dll': Ver=2.0.1.74, size=164112 bytes, created 02/11/2004 13:54:32
NT-SPTI used
Nero Version: 7.2.3.2
Internal Version: 7, 2, 3, 2
Recorder: <HL-DT-ST DVD-RW GCA-4080N>Version: 0C35 - HA 1 TA 0 - 7.2.3.2
Adapter driver: <IDE> HA 1
Drive buffer : 2048kB
Bus Type : default (0) -> ATAPI, detected: ?
CD-ROM: <HL-DT-ST DVD-RW GCA-4080N>Version: 0C35 - HA 1 TA 0 - 7.2.3.2
Adapter driver: <IDE> HA 1
=== Scsi-Device-Map ===
DiskPeripheral : SAMSUNG MP0804H atapi Port 0 ID 0 DMA: On
CdRomPeripheral : HL-DT-ST DVD-RW GCA-4080N atapi Port 1 ID 0 DMA: On
=== CDRom-Device-Map ===
HL-DT-ST DVD-RW GCA-4080N D: CDRom0
=======================
AutoRun : 0
Excluded drive IDs:
WriteBufferSize: 83886080 (0) Byte
BUFE : 0
Physical memory : 1246MB (1276336kB)
Free physical memory: 734MB (752384kB)
Memory in use : 41 %
Uncached PFiles: 0x0
Use Inquiry : 1
Global Bus Type: default (0)
Check supported media : Disabled (0)
14.8.2006
ISO compilation
19:51:03#1 Text 0 File Isodoc.cpp, Line 6427
Iso document burn settings
------------------------------------------
Determine maximum speed : FALSE
Simulate : FALSE
Write : TRUE
Finalize CD : TRUE
Multisession : FALSE
Burning mode : DAO
Mode : 1
ISO Level : 2 (Max. of 31 chars)
Character set : ASCII
Joliet : TRUE
Allow pathdepth more than 8 directories : TRUE
Allow more than 255 characters in path : TRUE
Write ISO9660 ;1 file extensions : TRUE
19:51:03#2 Text 0 File ThreadedTransfer.cpp, Line 531
ReadBuffer-Pipe got 81920KB of Memory
19:51:03#3 Text 0 File Reader.cpp, Line 124
Reader running
19:51:03#4 Text 0 File Writer.cpp, Line 113
Writer D: HL-DT-ST DVD-RW GCA-4080N running
19:51:03#5 ISO9660GEN -11 File Geniso.cpp, Line 3312
First writeable address = 0 (0x00000000)
19:51:03#6 Text 0 File Burncd.cpp, Line 3400
Turn on Disc-At-Once, using DVD media
19:51:04#7 Text 0 File DlgWaitCD.cpp, Line 289
Last possible write address on media: 4173823 (927:30.73, 8151MB)
Last address to be written: 3673791 (816:23.66, 7175MB)
19:51:04#8 Text 0 File DlgWaitCD.cpp, Line 301
Write in overburning mode: NO (enabled: CD)
19:51:04#9 Text 0 File DlgWaitCD.cpp, Line 2715
Recorder: HL-DT-ST DVD-RW GCA-4080N, Media type: DVD+R9 Double Layer
Disc Manufacturer ID: MKM, Media Type ID: 001, Product revision number: 0
Disc Application Code: 0, Extended Information Indicators: 0
19:51:04#10 Text 0 File DlgWaitCD.cpp, Line 473
>>> Protocol of DlgWaitCD activities: <<<
=========================================
19:51:04#11 Text 0 File ThreadedTransferInterface.cpp, Line 847
Setup items (after recorder preparation)
0: TRM_DATA_MODE1 ()
2 indices, index0 (150) not provided
original disc pos #0 + 3673792 (3673792) = #3673792/816:23.67
relocatable, disc pos for caching/writing not required/ required, no patch infos
-> TRM_DATA_MODE1, 2048, config 0, wanted index0 0 blocks, length 3673792 blocks [D: HL-DT-ST DVD-RW GCA-4080N]
--------------------------------------------------------------
19:51:04#12 Text 0 File ThreadedTransferInterface.cpp, Line 1059
Prepare recorder [D: HL-DT-ST DVD-RW GCA-4080N] for write in CUE-sheet-DAO
DAO infos:
==========
MCN: ""
TOCType: 0x00; Session Closed, disc fixated
Tracks 1 to 1:
1: TRM_DATA_MODE1, 2048/0x00, FilePos 0 0 7523926016, ISRC ""
DAO layout:
===========
. __Start_|____Track_|_Idx_|_CtrlAdr_|_RecDep__________
. 0 | lead-in | 0 | 0x41 | 0x00
. 0 | 1 | 0 | 0x41 | 0x00
. 0 | 1 | 1 | 0x41 | 0x00
. 3673792 | lead-out | 1 | 0x41 | 0x00
19:51:04#13 Text 0 File SCSIPassThrough.cpp, Line 39
SPTILockVolume - completed successfully for FCTL_LOCK_VOLUME
19:51:04#14 Text 0 File SCSIPassThrough.cpp, Line 84
SPTIDismountVolume - completed successfully for FSCTL_DISMOUNT_VOLUME
19:51:04#15 Phase 24 File dlgbrnst.cpp, Line 1730
Caching of files started
19:51:04#16 Text 0 File Burncd.cpp, Line 4231
Cache writing successful.
19:51:04#17 Phase 25 File dlgbrnst.cpp, Line 1730
Caching of files completed
19:51:04#18 Phase 36 File dlgbrnst.cpp, Line 1730
Burn process started at 2.4x (3,324 KB/s)
19:51:04#19 Text 0 File ThreadedTransferInterface.cpp, Line 2707
Verifying disc position of item 0 (relocatable, disc pos, no patch infos, orig at #0): write at #0
19:51:04#20 Text 0 File DVDPlusDualLayer.cpp, Line 470
Disc finalized, LBA total Image size: 3673792 (7175MB), Last LBA L0: 1836911 (3587MB)
19:51:36#21 Text 0 File DVDPlusDualLayer.cpp, Line 635
L0 Data Zone Capacity set to 1836912 (1C0770h, 3587MB) -> OK, confirmed setting
19:51:37#22 CDR -1205 File DVDPlusDualLayer.cpp, Line 662
Burning Layer 1
19:51:37#23 Text 0 File DVDPlusDualLayer.cpp, Line 672
Start writing layer 1 at address 0 (0h)
Recording mode: unknown Recording Mode
19:51:37#24 Text 0 File Cdrdrv.cpp, Line 8995
---- DVD Structure: Physical Format Information (00h) ----
Media Type: 0, Layer: 0, Address: 0 (0 h), AGID: 0; Length: 2050
Book Type: DVD-ROM (0), Part Version: 1.0x (1)
Disc Size: 120 mm, Maximum Rate: <not specified> (F h)
Number of Layers: 2, Track Path: Opposite Track Path (OTP), Layer Type: recordable
Linear Density: 0,293 um/bit, Track Density: 0,74 um/track
Starting Physical Sector Number of Data Area: 30000 h (DVD-ROM, DVD-R/-RW, DVD+R/+RW)
End Physical Sector Number of Data Area: FCFFFF h
End Sector Number in Layer 0: 1F076F h (LBN: 1C076F h, 3587 MB)
Data in Burst Cutting Area (BCA) does not exist
Media Specific [16..63]:
00 00 00 4D 4B 4D 00 00 - 00 00 00 30 30 31 00 40 ...MKM.....001.@
25 25 37 0C 02 2F 63 02 - 2F 63 21 23 0D 0E 14 14 %%7../c./c!#....
02 01 01 10 00 21 23 0D - 0E 14 14 02 01 01 10 00 .....!#.........
19:51:37#25 Text 0 File DVDPlusRW.cpp, Line 650
Start write address at LBA 0
DVD high compatibility mode: Yes
20:10:34#26 CDR -1206 File DVDPlusDualLayer.cpp, Line 1947
Burning Layer 2
20:10:35#27 Text 0 File DVDPlusDualLayer.cpp, Line 1958
Perform layer 1 to 2 jump before writing at address 1836912 (1C0770h)
20:11:17#28 SPTI -1039 File SCSIPassThrough.cpp, Line 291
D: CdRom0: SCSIStatus(x02) WinError(0) NeroError(-1039)
Sense Key: 0x04 (KEY_HARDWARE_ERROR)
Sense Code: 0x09
Sense Qual: 0x01
CDB Data: 0x2A 00 00 1C E2 E0 00 00 20 00 00 00
Sense Area: 0x70 00 04 00 00 00 00 10 35 01 00 0E 09 01
Buffer x057d0000: Len x10000
0x00 00 01 BA 44 17 44 1E AD 71 00 52 63 F8 00 00
0x01 E0 07 EC 80 01 03 1E 61 02 0D 4D 77 8C C4 23
0xE3 C0 DD 80 B9 77 D6 6C 9C C0 3F D7 9A 11 C0 FB
20:11:17#29 CDR -1039 File Writer.cpp, Line 302
Focus or tracking error
D: HL-DT-ST DVD-RW GCA-4080N
20:11:17#30 Text 0 File ThreadedTransfer.cpp, Line 228
all writers idle, stopping conversion
20:11:18#31 Text 0 File ThreadedTransfer.cpp, Line 222
conversion idle, stopping reader
20:11:18#32 SPTI -1039 File SCSIPassThrough.cpp, Line 291
D: CdRom0: SCSIStatus(x02) WinError(0) NeroError(-1039)
Sense Key: 0x04 (KEY_HARDWARE_ERROR)
Sense Code: 0x09
Sense Qual: 0x01
CDB Data: 0x35 02 00 00 00 00 00 00 00 00 00 00
Sense Area: 0x71 00 04 00 00 00 00 10 35 01 00 0E 09 01
20:11:20#33 CDR -1039 File WriterStatus.cpp, Line 167
Focus or tracking error
D: HL-DT-ST DVD-RW GCA-4080N
20:11:20#34 TRANSFER -24 File WriterStatus.cpp, Line 167
Could not perform end of Disc-at-once
20:11:20#35 Phase 38 File dlgbrnst.cpp, Line 1730
Burn process failed at 2.4x (3,324 KB/s)
Existing drivers:
File 'Drivers\PXHELP20.SYS': Ver=2.03.28a, size=20640 bytes, created 26/10/2005 21:12:48 (Prassi/Veritas driver for win 2K)
Registry Keys:
HKLM\Software\Microsoft\Windows NT\CurrentVersion\WinLogon\AllocateCDROMs : 0 (Security Option)
================= End of Log ===========================
I then tried reverting to the original firmware 0C09 from the DB site as I had read that some people were have DL success with it. With 0C09 Nero InfoTool reports DL read capabilities only.

So I think I'm now at a stage that I've tried all the things suggested in this thread.
Does anyone have any suggestions?
Cheers
Mark