Our wait turned out to be just under 31 hours.
The
idiot vet used the wrong tapeworm treatment, even though we'd asked twice to make sure he was using the right stuff, so when we got to the ARC around 10:15am (my flight came in at 7:35am, but we figured we were going to have to wait around anyway, so we took our time) they were ready, all right - ready to tell us that they'd have to keep them for another 24 hours and treat them with the right medicine, and that it'd cost us another £307.
Hopefully the same thing won't happen to you, but it's something to be aware of. We had a six-hour drive ahead of us, but we were lucky in that we were in our own car, had friends in Earlsfield with whom we could stay overnight, and Chris didn't have to be back at work until two days after my arrival, instead of the next day. The best-laid plans, and all that...