Dave
Interesting, I spoke to Brett @ BHM this morning, he was mistified as the only problem they usually have is the key not communicating - something I know you have written extensivly about.
But as inserting the key stops the flashing light that doesnt seem to be the problem.
However I'm not sure about the immobiliser, the flashing light goes out when I insert the key - tick
But if I leave the key in it stays out, whereas I'm sure that after about 20secs it is supposed to start flashing again
As I recall, the immobiliser light has a different flash sequence if you leave the key in but don't start the engine.
Try leaving the battery disconnected overnight, and also
make sure it's fully charged, there may be a voltage sensitive issue going on.