There are companies that will hold money in trust for you until an agreement is reached on completion of work, which is the arrangement I should have been using.
If you use an escrow service make sure you pick it, not the other side. If you go with a suggestion made by them (or anyone else) do plenty of background research before making any arrangements - there are some bad escrow companies out there and dodgy businesses (and individual scammers) are certainly not above creating a fake but official looking services.
There are companies that will hold money in trust for you until an agreement is reached on completion of work, which is the arrangement I should have been using.