Loss of water pressure on bathroom hot tap with down and up feed

[bide]

Bathroom basin has an hot tap where the feed comes down from the hot water tank in the attic to the floor and then up a pedestal to the basin. The feed is all 15mm copper.
When the tap is turned on there is full pressure but slowly the pressure drops until there is only a trickle of hot water. The feed is prone to airlocks but has been thoroughly flushed. The tap used to work well so what is likely to be the problem? If it is a fault in the tap what kind of fault could it be? Any ideas anyone.

 

[Chuck]

Description fits an almost-blocked supply to the the hot water tank. There may be a gate valve that has been closed for some reason and has not been properly opened. A crushed pipe due to someone careless standing on it while working in the attic is another possibility along the same lines.

A bit more information about how the water gets to the HW cylinder and where it comes from would be helpful. A photo is worth a thousand words. Did this problem come on suddenly or get worse over time? Does the tank supply any other hot taps and, if so, how are they working?

 

[Best]

I agree with above ^^^. And you shouldn't have any airlocks if hot cylinder is above and piped correctly.
Another couple of possibilities is (1) - a blocked airator (at end of tap spout), but usually only get airators on mixer taps and flow would stay constantly poor.
Or (2) - faulty works in the hot tap which will only allow the washer to raise slightly, to give good flow until the hot water reaches the washer and expands it to close water flow down. This is a very common fault on hot tap works

 

[king of pipes]

Have any valves been turned off i say this as the cheap brass red handled gate valves used on feed pipes to the cylinder are prone to snapping when closing them ?, i also had a similar problem a while back some fibreglass lagging had fallen into the header tank and blocked the tank outlet check this it should have a secure fitting lid, vent and a lagging jacket . regards kop

 

[bide]

Thank you all; checked everything as suggested. It's a faulty tap. Not really what I wanted to hear since, as I expected, was something of a nuisance to take apart. Still a good opportunity to upgrade all the fitments! Many thanks.
PS Amazing how many things could go wrong!