##UPDATE##
I sent my BCU off to Rick after removing it from the car whilst it wasn't immobilised and he tried a number of times to read it but with no success. In the end, he took the chip out of the bcu and soldered it into another known good bcu.
I received it back yesterday and fitted it back to the car. It started ok and the indicators worked in both directions. After running the engine for approx 2-3 mins, I noticed that the crossed "B" next to the mileage was flashing. I tried the indicators again and they had stopped working. I turned off the engine and everything was dead. No immobilisor response, no central locking or alarm and no indicators.
Since then I've tried leaving the ignition on for 5 minutes to reset the connectionion between bcu and idm, no joy. Tried battery off overnight and reset again today, still no joy. I've now fitted another IDM/Fuse box, reset for 5 mins again but I'm still getting the flashing B and no BCU response.
Any suggestions?