Hi again,
Response from Centon suggests this: "MacBook Pro EFI Firmware Update 1.6 "
http://support.apple.com/downloads/MacBookPro_EFI_Firmware_Update_16.
I don't think it applies.
Model Name: MacBook Pro
Model Identifier: MacBookPro4,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.5 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 6 MB
Memory: 3 GB
Bus Speed: 800 MHz
Boot ROM Version: MBP41.00C1.B00
SMC Version: 1.28f1
Still at a loss as to why both modules won't work together. Centon advises that system should boot with only one module installed, I'll give it a try.
Many thanks