Just as an update, I have now got all my accounts in order!
It has taken a bit of head scratching, and a fair bit of staring at the screen with my head in my hands, but I got there. It was complicated by a misunderstanding where some credits (a couple of overpayments) were applied randomly[1] to different accounts which didn't help.
I now have a spreadsheet showing the renewal date for each account and their payment status, and I pre-pay for 12 months hosting. I'd still prefer a proper annual payment system, but I can live with the new way.
All credit to the FQ staff who have done everything in their power to help, and have very cheerfully made manual adjustments for me where I needed to move credits from one account to another. Without their help, and the information that Questadmin makes available, I'd probably have given up in despair.
[1] It probably wasn't random, but it confused the hell out of me!
John