Worcester 24i Combi Hot water cmes then tap slows right down, why?

seem to be having trouble getting hot water upstairs in bathroom from a Worcester 24i combi. I turn the tap on slow, then as hot water starts to flow from the tap the water starts to run to a very slow trickle. So i turn up the the tap a little more, hot water continues, then after a few seconds water gets a little faster and cold water comes from tap.

Downstairs in kitchen seems to have no problems what so ever.

this is the best i can explain it. Help is appreciated.

Thanks.