The general case - not with a combi I believe - is that a boiler just heats the water - it neither knows nor cares what the hot water is for.
Usually an external valve is used to send the heated water to either a coil in the hot water strorage tank or directly to the radiators - many installations have the ability to share the heated water between the two - when both are being requested at the same time.
A Mid Position or Three Port valve does this - yours may be stuck in the wrong position.
They sometimes look like this...
http://www.toolstation.com/images/library/stock/webbig/60151.jpg
I am not experienced or qualified to offer advice on boilers - no warranty is expressed or implied.
You may need an engineer.
Good luck...
Peccavi
May 2010
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