Worcester WR325 Water Heater Not hot enough?

Hi, Our water heater isn't getting as hot as it should be, we have taken the front panel off and vacced it out and checked for anything obvious.
The pilot is lit, when hot water is turned on you see the flames appear and water gets quie hot, but then it goes down to luke warm, with no apparent diminishment of the flames.
There is a lead that is just twiseted onto a point which i will solder on just in case, but i think its something else. Please help, xmas int he cold isnt nice.