Welcome to the Two-Sixties! Your problem is more than likely down to water in or around the BCU I believe it's behind the glove box. Check the carpets in that area. If there wet there's a good chance you've found the problem. If there dry you may have just caught it. Either way the guys are right in saying it may be an idea to join the forum and unlock lots of great info on the cars.
How long have you had the car?
Regards
Nigel Baines
MG ZT 385 SE Starlight Silver Reg 29 Sep 04 Vin No 496 `Roush Supercharger fitted 07 Feb 11`