In reply to lone:
I'm assuming its a combi boiler? And does it do it at all hot taps in the house? How about the cold taps?
It could be a scaled up plate heat exchanger.
Could be a clogged up filter on the cold water inlet to the boiler, if it has one.
These would reduce flow, but probably all hot oulets would have the issue.
If its just that one tap it could be blocked aerator at the tap outlet, or knackered washer/cartridge. Or the washer growing as it gets hot, but this only usually happens when the tap's not very open to start with.
Wouldn't be pressure vessel related unless you have a mains pressure unvented hot water cylinder, which is a different type of system.