First suggestion would be to start at the beginning and disconnect all the wiring to the new stereo. You might find that everything goes back to normal. However if it doesn't and you believe that the BCU is really at fault then just buy a secondhand one from either eBay, one of the forum traders or a scrapyard, ideally with the same part number. If everything is fine then you can reprogramme the BCU yourself with Toaf (use the search for more info).
Regards
PS See that you're not that far away from
http://www.mjnmotors.com/ - said to be a top bloke who can sort out your BCU, (but I'd still start with the stereo).