Problem with burner/temp and heating water on Worcester Combi gas boiler

Levitz

I have a problem with my Worcester Greenstar 35 HE Plus Combi gas boiler heating the hot water. I live in a flat and rarely use the CH, and have the boiler set to economy mode, when I turn the tap on and 'call' for hot water the burner starts up as normal and starts to heat the water but very quickly it says it is heating it to over 80 degrees and the burner cuts out and once it goes past 99 and the trap filling sign comes on the display. Once the temperature reads below the max flow temperature and I 'call' for hot water again the burner will go on again. But I am only getting a few minutes worth of hot water at a time. The pressure is above 1 bar but in the green zone (ie acceptable). I shut off the water stop !!!! over Christmas as one of my radiators had had a leak and wanted to ensure I didn't come back to a flooded flat and also switched the boiler off. I had already turned off the radiator, but it was a belt and braces exercise to stop me worrying while I was away.


Can anyone help with what the problem might be, how to fix it and (if an engineer is necessary) how much it might cost.