Many thanks for your answer @RoseJillB , I have been able to use all of your advice.
However, I would have got to that stage much quicker had you started with one simple clarifying fact:
GoCardless payments appearing under 'For Review' means something has gone wrong with the automated process.
I did not know this; I thought all GC payments arrived into 'For Review' and was baffled as to what to do with them. The option to Add was there, I was just certain it was not the correct action, and changing it to Match instead of Categorise also looked wrong. I felt stupid for not not being able to process a simple transaction. I scoured the articles and community Q&A and did not find a single reference to my situation (GC payments in 'For Review') or any mention of the automated process not completing, or how to trouble-shoot it.
For anyone else in this situation, the answer is to export/download the Payout transaction from GoCardless and compare the entries in that to the entries in your Undeposited Funds account. Had I known that yesterday morning, I'd have been finished by lunchtime rather than 8pm. Based on my experience of dealing with 7 working days worth of GoCardless Payouts (>1 per day), the automated process fails in 4 scenarios:
- Euro payments - fair enough, you need to tweak the exchange rate to get the pennies right;
- Extra debits - specifically for failed DDs (don't know if there are other reasons). Again fair enough, that needs attention, and the resolution in the Deposit is easy enough.;
- Invoice is not open - again fair enough. If the customer has already paid the invoice by BACS/FP then also by DD, you need to know so you can refund them. You enter the sales payment on account and take it from there;
- Reference is anything other than "Payment for QuickBooks Invoice xxxx" - this one is not fair enough. Even a reference of "Invoice xxxx" is insufficient. IMHO, a valid mandate and a valid (open) invoice number should be sufficient.