I've used Medway Galvanising in Sittingbourne, Kent. There was a minimum weight requirement, but I exceeded it with a chain and two anchors. Not sure of the system, but they did a good job with the chain, no sticky links.
When we were there in may (so may be different now) there was a very helpful trot boat service that operated weekend evenings. This was free but I was under the impression that the operators was paid by the club or the trust. The day time crew were rather less helpful.