How do I fix water hammer just started on closure of hot water valve?

My central heating system (vented, not sealed - boiler replaced about 4 years ago) has suddenly developed a loud bang when the valve on the hot water side of the system closes. This has been going on for a couple of months and there was no problem before. The water valve was replaced about 2 years ago and was fine until recently. It has a Honeywell actuator and the valve seems to operate on a quarter turn. The central heating side seems ok. There have been no changes to the system recently. Why would this problem suddenly develop and what can I do about it?

David
March 2010