what time do you think you'll get there Mark? I'm not at all sure about late opening hours for the AP office in the park, but it was after 7.30pm on a weekend that I got my renewal AP from Bureau Donald last time.
Might be worth giving DLP a call or an email to ask where you can purchase/renew your APs after 7pm (this is the normal closing time for the AP bureau I think).
Its literally opposite the side door to the Disneyland Hotel (so you'd have Guest Services and Salon Mickey ahead of you as you approach the main park, side door to DLH is on your right, Bureau Donald is on your left).
I had no problem at all renewing my AP in full at Bureau Donald, I didn't have to go to the AP office in the park, it was laminated etc for me at Bureau Donald.
Thanks, I might bite the bullet & purchase an AP this time but don't want it to eat into park time so may pop back to get it whilst the kids & my sister are resting