The inspector very kindly offered to come today (a Sunday) so that I could be there when he inspected, telling me not to jump to any conclusions in advance. The weather wasn't great but we managed to get everything done before the drizzle set in for the day.
He inspected the other four hives first, gave them a clean bill of health and also gave me some helpful advice on what I could consider doing with a couple of them.
We then came to the smelly hive. Embarrassingly the cheesy smell had gone.... He painstakingly inspected both boxes, opening and checking individually all the cells with perforated cappings. Nothing untoward found - a good strong colony.
He dismissed my apologies for calling him out, as BJ said he would, saying he was happier to report a foulbrood-negative result than a positive one. When I asked him what he thought had caused the smell he didn't mention dandelion but suggested it must have come from what they were foraging on. Sure enough, the field next to the apiary has been covered in them. (Hats off to BC, ITLD and CB!)
He was very happy to answer all my questions and I found the hour spent watching and chatting to somebody who knows what they're doing invaluable.
The whole visit was a very positive experience, I thought the inspector was excellent, and I'm delighted to settle for cheesy-tasting honey rather than the loss of my bees!
Other things I now think I should have considered:
The hive is strong (see op - today it had 19 seams of bees and 10 frames of brood) and was unlikely to be in the terminal stages of foulbrood, when the stink is likely to become evident (as pointed out by RB.)
When inspecting I should check frames more carefully, opening individual suspect cells rather than just relying on a general impression.
The reason that only that one hive smelled was because it is so much stronger than any of the others and had brought in much more nectar.
There isn't much foulbrood in our area apparently and as I haven't brought in either equipment or new bees from anywhere else recently and as far as I know there aren't any other beekeepers close-by, there wasn't an obvious vector for infection.
Other lessons to add?
Thanks to all for your input.
