alex59, I don't quite understand your logic - why exactly are you expecting to do a 50-hour check 4 weeks before the annual?
In any event, according to CAP766, you are allowed up to 10% extension on a 50-hour inspection - that is, you can defer it by up to 5 hours extra. On the other hand, you are allowed a 30-day variation on an annual, too, so you can just do the annual 4 weeks earlier than you are required to, and it won't affect your next year's annual.