Yes, this has a check valve but even if faulty or the valve has been installed wrong way round, the cylinder will still be supplied from the shower supply (which in shouldn't), just shutting that red lever valve will prevent this and also prove that the cylinder is being correctly supplied.
Personally, before doing anything more and in view of flows back to normal this morning, I wouldn't spend another cent but I would go away and invest in a £10 pressure gauge that you yourself can screw on to your outside tap and observe it for a few hours or so, its so simple and basic to do this IMO.
Personally, before doing anything more and in view of flows back to normal this morning, I wouldn't spend another cent but I would go away and invest in a £10 pressure gauge that you yourself can screw on to your outside tap and observe it for a few hours or so, its so simple and basic to do this IMO.