... and we only found out when they had left and we were trying to let ourselves in to do the final inspection.
They didn't ask permission and didn't inform us after the fact (apparently one thought he sent an email, but has admitted it didn't actually get sent). They have our mobile numbers and don't hesitate to text if something goes wrong (and we fix things immediately - having rented for years we try hard to be good landlords). So they definitely had a way to get in touch but chose not to use it.
They finally dropped off the new keys with the estate agents a few days after the move-out date, which caused us massive problems because we didn't have the weekend to prepare the property for the new tenants. No apologies at any point, and rude responses to our emails.
I have calmed down from the initial panic that we couldn't get into our property, and now am trying to figure out what to do about this. It is a clear breach of contract to have changed the locks without informing us, and also handing back the keys late is not only a breach of contract but also caused us a bunch of extra inconvenience. Estate agent (who does not manage the property, only finds new tenants) has said any deposit withholding is up to us.
We will definitely deduct the cost of new keys we need to get cut, but how much would be reasonable to withhold on top of that? If anyone knows of official guidelines I'd be grateful to hear about them. We would like to be good landlords but - quite apart from the initial heart attack when we were trying to find out what was going on and why we couldn't get in - this seems like the kind of crappy behaviour tenant deposits are designed to discourage.