Low flow/pressure on hot water

My hot water setup is currently an S-plan system with an unvented cylinder, heated by a gas boiler. The boiler works correctly, heats the water no problems for both the DHW and CH circuits. A new pump and expansion vessel have recently been fitted, the water circuit holds pressure with no evidence of leaks anywhere.

When the water tank has cold water in it, the pressure is close to what comes in from the mains.

When the boiler has heated the water to 55°c, then thermostat in the cylinder makes the boiler switch off as it should, and there is lots of nice hot water in the tank.

My problem is that when the boiler has heated the hot water, there is almost no pressure on the water from the hot taps. Upstairs is worse, downstairs only slightly better. This is most obvious when the boiler is actively heating the water, it comes out the tap under huge pressure and blasts everywhere, then almost immediately slows to barely a dribble.

Once the tank has been heated and the boiler has been off for a while, the got water comes out under pressure just fine.

I guessed it may be a blockage in the pipework, sludge in the cylinder, or a blocked filter on the PRV - but why would the pressure on the hot water taps be okay when the water is cold? A faulty mixer tap perhaps?

Im guessing that when the water is heated it will expand and this is what's causing the initial high pressure blast of water from the hot tap... but thats about as far as I can figure out.

Would really appreciate anybodys help!