Thankyou for your help here, the issue is now resolved.

I thought i had read this in here, but the error in this scenario was related to a resistor under the battery, it was completely corroded and had failed. This sounds simple, but be aware, the battery weighs just under a tonne, so its not easy to remove. Once it is lifted out, there is a chunky, 120w 1R5 resistor under it. Toyota claimed nearly £800 for a replacement, with a 3 month waiting list - I ended up going to a company called Farnell (uk.farnell.com) and buying one for £20 (MPQLJ015J120).

The strange thing i dont understand is why its there. The wires reach out to the CPU board, so i just stretched it out there, put a casing around it and it all works (after the nightmare of getting the battery back in..!)

All in all, thanks everyone for your help, if this can help someone else then im happy!