Brilliant thanks for all the detailed information.Rules to Follow to Avoid Pump Problems | Valin - https://www.valin.com/resources/blog/rules-follow-avoid-pump-problems
The pump should be located close to where it draws suction from the cold water tank to ensure the minimum Net Positive Suction Head Available NPSHA of 2m h2o Is exceeded. If a long thin pipe run provides the water feed to the pump inlet there is the risk of the NPSHA dropping below 2m because of friction in the pipe, its bends and joints and turbulence acting to reduce head of water pressure.
The cold water tank also provides a large reservoir of water to provide a short term high flow rate to the shower which is why I do not recommend connecting the pump to your mains water pipe directly. At a high flow rate the water pressure in the mains pipe coming into your home will decrease and thus could fall below minimum inlet head of water of the pump.
Note also that you should avoid sharp bends near the inlet and outlet of the pump. Have long arcs of pipe work so the water flow is not overly disturbed.
I'll keep the pump set up as it is.
I do believe there was some air in the piping the other day after changing a non return valve on the immersion heater tank on Friday though. Appears to be functioning better on the upper floor bathrooms now. Still the hot water pressure is not great from the upper floor immersion heater. I wonder if it needs an additional smaller booster pump in the airing cupboard where the immersion heater is. That non return valve I believe causes the issues initiating the flow sensor