I've got all the hives rearranged on proper stands now. The swarm checks out as 'near-native', meaning that most of its characteristics are those of native UK bees, but there's a bit of a discrepancy here and there. In this case, the bees are very slightly banded, and the body hairs are too short. The bands are brown rather than yellow, and, importantly, the characteristics are consistent, being the same in every individual bee. That's close enough to a native stock to be worth keeping. I still need to check out the new colonies - a lot of the bees have yellow bands, indicating hybridisation, but that doesn't necessarily mean the other characteristics aren't right. Then I need to watch them next summer and see if they're showing signs of mite resistance.
My own strain detect mites in capped brood, open the cells, and clean out the larvae with the mites. Some strains groom the mites off each other, and damage them in the process. These aren't characteristics you want to lose through careless breeding!