I think I know what the answer to my question will be, but just in case the situation is allowed for I thought I'd check.
My Y-P railcard expires on 15 May 09 (which is a Friday). Sadly, I'm now too far over the hill and too long in the tooth to renew it

As it happens, I'm going away over that weekend. The booking office presumably will sell me an off-peak return ticket for my outward journey on the Friday, when my railcard is still valid. However, by the time of my return journey on Sunday 17 May I won't have a railcard any more or be eligible to renew it.
I assume this means that I can't use the railcard at all on the off-peak ticket, but if there are any rail staff out there who know different then please tell me!