Shower pressure slowly dropping

I live in an apartment and I'm having problems with the shower. The problem started when we had low system pressure in the entire block. This returned within a few hours and each tap/source we'd used needed to be run to clear the air from the system.
However, the mixer shower has not fully recovered. If I leave the shower off for a long time, there is something approaching system pressure when I initially turn the shower on, but this soon drops to just a trickle.
I presume there is an air blockage somewhere in the system but I don't know how to solve this problem. No other taps in the flat are now affected and the hot/cold water mix of the shower is fine, just the overall flow rate drops to nearly nothing within about 5mins of use (best-case).
Any suggestions?
The mixer-shower is built into the shower cubicle. It has a MIRA head and temp/flow control valve but no easy access to it.
There is good access to the boiler (Classic Tribune) and flow circulator (Grundfos Super-selectric) and Y-style valve (Lardis and Gyr) if this will help, but I fear the problem is unique to the plumbing at the shower and none of these. The central heating works fine and all rads are fully bled etc. and hot/cold water is available at all other taps with full system pressure.
Please help! Many thanks
Tom

5 mins is a long time before it starts to reduce flow.
you shaw it's air in the pipework and not the tank in loft not filling quick enough

I tried it again this morning and it lasted at the normal pressure for about a minute and then slowly reduced to a trickle at 5mins.
We don't have a loft as such, the hot water tank and circulator pumps are all on the same level as the shower. Until the water shortage last week the system pressure in the mixer shower was very good.
Would it make any sense for me to manually exercise the Y-style valve? Could it be that this valve is not fully allowing hot water to the mixer shower and hence the mixer unit is reducing the permissible cold-water flow aswell?
Having said this, I have tried running the shower with just cold-water and it still shows the same problem so I don't think it is related to the hot-water feed- it seems to be much more local to the shower itself. I was able to run the hot-water tap in the same room for 10mins this morning with no drop in pressure.

Hi, check the ballvalve on the cold water storage tank and make sure its flowing well.
The cold water storage tanks in apartements are normally around 25 gallon, and don't forget, this is supplying the cold to the shower, and the hot water cylinder. I tend to agree, it points toward a lack of water supply rather than air.
The valve will simply shut down once starved of either hot or cold.
I hope this helps.

This may sound stupid but I genuinely do not know where the cold storage tank is located. Would this tend to be something external to a single apartment, i.e. shared between perhaps a floor of apartments?

I ask because the only access I have in the flat is to the items already mentioned, i.e. the (huge!) hot-water tank and circulator etc.

If it is a problem with the cold supply to the flat, would the flow problems not also be seen at all other taps? The pressure in these is superb, the trickle coming out of the shower after a few mins is the only evidence of reduced flow in the apartment.

Sorry I'm not able to provide more detailed info- this is the first flat I've had hence the lack of understanding of the overall typical system.

Out of interest, does the circulator only tend to be active for circulating the central heating water? I've not noticed this pumping but we don't tend to use central heating anyway- just checking that this isn't typically part of a powerered shower supply for example?

many thanks
Tom

Normally in an apartement or flat the cold water storage tank is in the same cupboard as the hot water cylinder, (directly above it), or there may be one big tank supplying a few of the apartements...

There are a number of pipes coming into the flat- I'm reasonably happy that I've identified the overall cold water source (insulated pipe coming in through the ceiling from the floor above) so I will trace this tonight (there must be a water storage tank located up there somehow).

But I'm still not sure what I'm looking for on this tank really as to my mind this doesn't explain why the flow is only affected in the mixer shower?

Thanks

If the tank is communal, then I doubt its this that is the problem.
The shower valve may need looking at closely.
Check the filters (can normally be found on the feed pipes) are clear of debris. If they are it may be worth getting a new integral cartridge for the valve.