This is what we did in the end.
The buyer came up on saturday morning. Once he had arrived, we did the paperwork but left off the final bit confirming sale had completed. His partner in Oxford did a bank transfer for half the amount, via her phone app. This should have taken a maximum of 2 hours, but it ended up taking 3. Nail biting times. Her bank told her she wouldn't be able to transfer the second half until sunday.
I'd promised the buyer I'd travel with him the first couple of days, so we set off early afternoon from Hawne Basin and moored saturday night in Bournville, just before a biblical rainstorm. I left him with the boat but kept the engine starter key.
I woke up sunday morning to see the second payment had gone through (transfered at midnight), so went back to Bournville, filled in the final bit of paperwork, and helped him from there to Kingswood Junction.
His partner and their dogs arrived last night, and they're now on their own, heading for Oxford. If you see them (green boat, Beau on the side), say hi. They seem really nice and I'm sure Beau is in good hands.