Low water pressure in hot tap

A few weeks ago the water pressure in hot water tap(bathroom and kitchen) suddenly became a lot lower than what it had been. I thought it was some temporary problem and then was away for a few weeks, but it hasn't changed. ie turn the tap on, normal pressure for about a second then suddenly very low pressure.
Anyone any idea what might be causing this and if it's complicated to fix?

More than likely an air lock. There are quite a few threads on this and the solutions already. Try the search facility to locate these.
Leo