We are just about to fit a bathroom in off the main bedroom. Question is we have an electric shower in the main bathroom so don't think the electric supply would like 2 showers running at the same time so plan to fit a shower run from the hot water tank. The problem is the hot water tank is down stairs so the water pressure upstairs isn't brilliant so would it help if we ran an other cold feed from the tank in the loft to the hot water tank inlet so had 2 feeds coming together at the hot tank or do you think this would be a waste of time. Would prefer not to use a booster pump had one before and the noise was a pain.

the cold feed to the tank should be fine. The issue is the height of the cold tank. Could it be raised any higher? many are on raised platforms in lofts for this very reason.

If you are concerned re the flow, just have a look at a bath tap running. Usually all 22mm stuff, but as I said, I reckon it is pressure that is the issue.

Now, years ago you could buy a shower that used the mains cold water supply to improve the pressure of the hot supply, sort of power shower but powered by cold water pressure. No idea what they were actually called, or if still available, but worth a look.

search "venturi" showers

Can't you put another electric cable from the meter to the new shower ? it must be quicker and better then putting a new pipe to the hot/tank?

You can run a new cable from the consumer unit to a new shower. They have to have separate wiring anyway. We have had 2 electric showers in the past and it was fine.

For the gravity supply, the pressure of the hot water is dependent on the height of the cold tank in the loft. You cannot run a mixer shower with gravity hot and mains cold as the pressures would be too different. The usual setup is gravity hot from the normal bathroom hot supply and a separate cold supply direct from the cold tank.

If the gravity pressures are low you could fit a power shower - it has an electric pump to boost the pressures but the supplies still need to both be gravity feeds. The power supply does need to be the massive cabling required for an electric shower. You are supposed to use a dedicated take-off from the hot tank for the shower or else you risk 'sucking' air in from the expansion pipe. I have fitted one without and got away with it though. Google Surrey, Warix or Essex flange.

If you think you are likely to replace the heating system at any stage, and go for a combi, everything becomes mains pressure and you can fit mixer showers throughout, although having an electric shower is a good back up.