Unvented cylinder responsible for low flow rate?

Unvented cylinder responsible for low flow rate?

I have good mains water pressure on the ground floor.  There is no tank in the loft but an unvented Fabdec Solar cylinder in the upstairs airing cupboard. A gas heat only boiler (Vaillant) which regularly displays a F83 fault.

I've checked all the stop cocks and they are all fully open. The upstairs has a balance valve. There is a pressure top up in the airing cupboard and its at 1.5 bar.  Upstairs I have two en suite rooms.

The cold flow rate is very poor in both en suite rooms whether shower/bath/basin taps. Sometimes nothing comes out. The hot is good for about 30 seconds and then it drops to almost nothing in the shower/bath and basin taps. You cannot have a shower or bath at the current rate.

Could there be a problem with the valves on the solar cylinder?  The solar thermal panel was removed from the roof but the pipework is still there.  The solar controller and pump station are switch off.