Thanks for the replies, here are some answers to the queries raised together with additional background –
· My pump is on the Hot line,
· Tap & Cylinder are on the same floor, the height difference from the Top of the Tap to the Cold water Cylinder (attached to top of my cylinder) is about 900 mm MAX. – this is a fixed quantity
· When I removed the Nozzle from the top of the tap – It works (the resistance within the Nozzle/handle is enough to increase the flow above critical point) – I’ve measured the flow rate and yes I am on the edge of the working/Not-working figure.
· Tap came with “Multiple Non return valves”, (cold water, Hot water feeds, Spray head etc.). When I remove one from the HOT line leading up to the tap tail leads and the One within the Spray head itself, things did improve (left the cold in place – no problem here – lots of cold water coming out).
· I have two Isolating valves in the hot line between tap and the Pump. I’ve checked that these are Open, even thought of taking one of them out – Isolating valve, run of ½ inch copper followed by isolating valve not really necessary (historical)
· This Cylinder/Pump only services my Kitchen, rest of house does it’s own thing with a tank in the roof – no problems.
· Tap came with Long Tails ~ 400 mm, thought of replacing with shorter – any ideas