I think I've tried every trick in the book for this one and I'm having no luck. So frustrating the after an engine rebuild the alarm system is stopping me from driving!
It seems that, since I had the UK version of the Z3 there is no anti-theft system for the alarm (that was a US specific thing). I've tried reprogramming the fob, disconnecting the battery for a day etc - I just can't get the system to disarm. If I open a door/boot/bonnet the alarm goes off, if I move inside the car the alarm goes off, if I rock the car from outside the alarm goes off. It doesn't matter whether I have the keys in the ignition/the car running or anything.
Think I'm going to have to call a BMW specialist out on this one which is going to cost a fortune. I'll be sure to post the solution if/when I find it.