How to change the bathroom cold supply from storage tank to mains 

Keep the mains 15 mm.

If your shower is fed from a roof tank then you may need to put a mains cartridge in it. This video was just about those with the bath tap basin tap and WC being fed from the storage tank. The pipes to your shower would have been reduced to 15 mm to fit which can cause problems, but more than likely the cartridge was calling for less hot water and more cold, but the cold from the tank is, or may have a valve fitted somewhere causing a restriction in the pressure. Check for isolation valves.

@@dereton33 Thank you very much for that, I know I went on a tangent but it was sort of a similar subject. I'll certainly look closer at the system, seems like a DIY job...or a plumber was doing a favour for a friend and did as cheap as poss! Keep up the good work 👍

Sorry I am retired now, thanks for asking.

It was a popular council requirement back in the day, for new build houses. This meant that both the cold and hot were of the same pressure.

So long as your roof tank does not supply water to the cold taps and cistern in the bathroom.

Worth checking, But it more than likely is a blockage in the valve.

Not with a combi. No Jay.

It is the Hight of the roof tank, not where the cylinder is, that determines the pressure. That said, a pump is still the best option.

@@dereton33 what's weird is we used to have a hot water tank in the attic years ago. But some work was done in the airing cupboard upstairs to add an expansion valve etc and I believe the tank was disconnected (not been up in the attic, it need a good professional clean). I was young at the time so didn't know too much. All I know is the pressure has never been the same

I guess they have just got used to it.

It’s an old thing. Modern houses would take it from the mains. In the early days, water pressure would be erratic so it gave everyone a constant source of water (for baths and toilets etc). I think there are some other reasons too

A lot of councils insisted on them in the build.

The reason for the storage cisterns was rather more practical: The water bylaws for London and many other cities mandated them to ease demand on the supply at peak times. Imagine when everybody gets up in the morning and starts drawing off water for washing, making tea, using the toilet, etc., or the similar surge in demand which occurs when large numbers of people arrive home from work and start washing up after their day's toil, preparing meals, and so on. When the supply system is working at full demand, the pressure drops and the rate of flow to each house decreases. The storage cisterns act as a kind of buffer, since they will simply refill a little more slowly than at other times. In other words, you can draw water out of the tank faster than it's being replenished (for a limited amount of time, obviously), thus everybody can get their water but the supply system doesn't have to be made that much larger to cope with the peak demand directly. The cold tap for kitchen sinks was made an exception from having to be fed through the tank, to provide non-stored water for cooking and drinking.

@@davelinkson1 that's a much better explanation!

Cheers George.