I have a loss of hot water pressure from the hot tap in the kitchen after 30 seconds or so. It is fed from a combi boiler, any ideas?

Watter pressure.
"A" By volume large pipe (low pressure) a lot of water.

"B" By pressure: Smaller pipe (higher pressure) a lot of water.

A and B pipes will equal the same amount of water flowing in a bucket.

What changes that is "pressure"

A domestic coil in a boiler overtime can build lime deposits. Restrict the water flow.
Any restricted water line in system same result.

Did this just happen all of a sudden? Is there a shutoff valve on that line ? What type of faucets there? If valve seat or cartridge in faucet doesn't open fully same symptoms.
Could be a "T" fitting on the water line restricted on part of "T" same result
Please provide: water line info , copper, cpvc galv iron. Black iron, Hot line 3/8 1/2 style of faucet? Just this line?
More Info Please> Boat 21