Funny, I was in the same situation recently & was assured by several clubs I spoke to that it was the flying rather than the paperwork that had to be done before the expiry date...
They said that if you sent the documents with a covering letter to the CAA with an explanation of the reason for the delay (out of country, sick, pressure of work etc etc) they are known to give you a dispensation.
Sadly that assurance is misplaced. The revalidation MUST be signed before the expiry date.
I'd be suprised if the CAA were to grant a dispensation. They allow the paperwork to be completed up to
3 MONTHS BEFORE the expiry date without any shortening of the revalidation period, to allow for unforseen circumstances.
The best advice is to get signed up good and early, give yourself peace of mind.
BTW, £75 might be an average test fee, I know of some examiners who charge as little as £30, others as much as £150. Revalidation signoff for non-club members is usually around £30, too. After all, it costs thousands to obtain and keep up an examiner rating, the money has to come back from somewhere...
TheOddOne