Hot Water problem on my 37CD1?

I have a Greenstar 37CDi gas combi. Only a few months old. Worked ok up to now. On heating side all is ok. On hot water however, despite turned up to almost max and in "on" switch position, when I open the kitchen tap hot water comes out to start with (although not very hot) and after about a minute the temp drops to luke warm. There does not seem to be anything that refers to this in the handbook and I am just wondering if there is something easy I can do before I contact the manufactureres