Central Heating but No Hot Water

[Abe]

I am having problems with my vaillant turbo max plus (I think) Combi boiler that is only 2 1/2 yrs old. The heating works fine but the hot water doesn't If i run the hot water tap very slowly then it comes out hot for a short while but if I run it at normal speed it comes out very cold. The pressure is fine but I can only get hot water If I run the tap really slowly. I am not an engineer but trying to get a plumber to look at it is a bit of a nightmare so I am prepared to try and do something myself if possible. any help would be appreciated.

[BigWallop]

Does the boiler fire up OK when the taps are opened? It could be something as simple as you needing to turn the thermostat up a bit.

[Abe]

The boiler fires up ok and I've got the hot water thermostat on the hotest setting. I think, reading some of the other similar post's its something to do with the heater exchange.