I h ave a combi boiler and have just had some taps changed so need to turn of the boiler and the mains water. Once done turned all back on , made the pressure on the boiler back to 2 and a half bar but now the water pressure coming out of all hot and cold taps is very low.. how can this be solved?

Look for debris. Might have knocked some dirt or calcium loose during the valve change. Make sure all valves are full open.