Went to check the rafts on the canal. There was nothing at all on the first one, so we buried it more thoroughly in the reeds (middle pic) and it'll be looked at again in a few days. The one further down had prints on but they're neither water vole nor mink - much too small for the latter. Stoat?
The surface of the clay was re-smoothed with some sort of plastering tool, and the raft put back into position. I get the impression these clay inserts can last for months or even years.