Mains Pressure Hot Water Cylinder Low Flow - Plumbers?

I have a newer water tank and expansion vessel. On mine I have a pressure gauge and valve I can open to bring the pressure back up to 1.5 Bar on the gauge. Do you have a similar setup?

I have no pressure gauge on the water circuit.
There is a pressure gauge in the boiler but that's for the rads and the heating coil not the main water storage.
@boardinbob : It does look like a lot of the newer valves do have gauges as you say.

My plumber just uses a track pump (with gauge) to top up the pressure in both the boiler and hot water cylinder expansion vessels. You probably have one so it's worth a try - mine both have schrader valves.

Is it a megaflo ?  They have an air bubble inside the tank that needs regenerating every now & then (is easy).  Not really sure whether your symptoms fit with that, though

Pressure and flow are two different things.  Sound like you  have plenty of pressure but low flow

pressure without an expandable component is pretty much "fixed" though TJ - at the setting of any relief valve.  Once you open a tap I'm thinking that there's no compressed air to maintain that push and so flow would drop pretty quickly.

IANAP or even a competent DIYer

They have an air bubble inside the tank that needs regenerating every now & then (is easy). Not really sure whether your symptoms fit with that, though

Nah, You can tell when a megaflow needs the air gap replenishing because there will be hot water flowing from the tank into the tundish and outside via the overflow pipe.

OP, do your cold taps have good flow?
If they do then it sounds like the flow into the bottom of the hot water tank has been reduced by something.
Check to see if there's any kind of a filter on there and that's the valve hasn't been partly closed for some reason.
As far as I am aware, there is no compressed air on these systems, just an expansion tank.
The whole idea is that the hot water is pushed out of the tank by the incoming cold water and is therefore at the same pressure as the cold water ..... hence why it is a balanced flow.

Is the bottom one a pressure reducing valve? could that be clogged?

Possibly clogged or just stuck.

If we put a thumb over the tap, the pressure builds and a good jet is obtained but flow then falls when thumb released. This I believe is the expansion tank expanding and relaxing.

That's simply you reducing the size of the hole the water is trying to get out of!

Oh, And is it the same at all the hot taps?

All taps have low flow rate once the initial surge has died down.

I think the PRV can be serviced and cleaned by unscrewing the plastic cover which is hiding away behind the expansion vessel in the picture.

pressure without an expandable component is pretty much “fixed” though TJ – at the setting of any relief valve. Once you open a tap I’m thinking that there’s no compressed air to maintain that push and so flow would drop pretty quickly.

That's not what the expansion bubbles for.

Id be looking at a malfunctioning Non return valve / blocked filter (often located together) and or a knackered/blocked pressure reduction valve / thermostatic blend valve if fitted

What Trailrat said, blocked strainer in the control group or the PRV failed in it, changed dozens over the years. Expansion vessel wouldn’t have that effect complete waste of time, it deal with expansion of water when it is heated, nothing to do with pressure and flow. If it fails you get discharge of water from safety valves, not low flow.

Ianap so remember this.
Inditect unvented cylinders are rated to 3bar max. Some mains supply can be higher so a Pressure Reducing Valve is fitted upstream of the cylinder.
As the pressure increases a spring slows or closes the inlet water.
There will be an prv pn the top of the tank, above the tundish, this is to protect the tank if the supply prv fails.
From the description it might be the supply prv is clogged, there is a filter and usually a guage.
See if you can find the supply prv, check the pressure and clean the filter