Oil Combi Boiler Help

I've got a 8yr old Wallstar oil fired combi boiler in my garage and I've noticed that the hot water is getting colder. When I moved in about 8 months ago, it was not really hot, but just about ok to have a bath without needing and cold water. I had a bath today and the water was only warm. The central heating side is very good and it just seems to be the DHW. The tap has to be run on full to get any hot water and then it is only warm. If you put the tap on half, it only runs cold. I've checked the boiler pressure and DHW temp settings and they are all ok. Any suggestions or do I need a call out??