They should all carry out checks, references and Enhanced DBS Checks - however, checks are only good for those who behave and have not been caught, so often these episodes will not raise any flags sadly! Any issues, then report the operative. You may also have a sub-contractor working in your home (housing associations to cut costs are subbing out a great deal of work and not necessarily to the best, but the cheapest!), so not from the housing association and they may not have been checked as thoroughly, but should have been. You should always ask to see their IDs and if you are not sure then don't let them in, especially if no prior appointment has been made and it should always be. Also do complain, as you are paying the wages of those that come out to do work on your homes, so you have every right to expect a good quality of work and behaviour, at all times!