sgk: I spoke to the Scottish B&O engineer on the telephone and he basically said that because it was so old it wasn't worth trying to fix it! Surely that can't be right......
And it isn't, this is the standard excuse they use when they don't want to hear about an older system anymore.
Most probably the memory backup battery in the Beomaster has run flat. Replacing it is a job for a technician, but nothing major.
You could also try keying 1-2-7-1-PLAY on your remote right after plugging the Beomaster in, which is supposed to reset the "code" display after the battery has been replaced. With a bit of luck, it will work again as long as you keep it plugged in.
See also HERE - different unit, but the same problem.