This nuthatch has given us the once over a couple of times before. I thought it must be getting late for it to be still investigating nesting sites, but Elizabeth checked in one of her reference books and nuthatches don't start nesting until the end of April. Anyway, the hole is too small for a nuthatch whenever they start nesting!
(According to the RSPB, nuthatches need a 32mm entrance hole and this one is only 28mm wide!)