Thank you terrier and hopalong for the links.
Three of my boxes are sited on east-facing fences. I don't have any other suitable locations for them, unfortunately. Wind and rain is usually from south or southwest so that shouldn't be a problem - but maybe the early morning sun is not to their liking. Also my bird feeders are not more than 3 metres from the fence as I only have a small garden!
A great tit was pecking round the entrance of one of them yesterday, so hopefully he will return.
Tricia