Thank you for your detailed reply and suggestions.
Cistern refill and tap use definitely worked completely normally at same time as bath cold was running prior to work. And bath cold ran with much stronger flow than now as well, so definitely a change.
The new tap on sink downstairs is mains...
My kitchen sink is mains cold and gravity hot, as is utility room. All others are gravity cold and hot.
Worth trying this on kitchen sink? Hot water all totally fine everywhere!
Downstairs basin cold is a weird looking waterfall style mixer tap with like a horizontal opening for the water to pour out from so I didn't think would be possible to get a decent fitting/connection from the hose onto it. Sorry I don't mean to have an excuse for all these helpful suggestions...
I'm not sure, I've borrowed it off a mate. It's a Charles (Henry's pal) wet vac if anyone knows if he has this function? Quick Google isn't showing me much info about how to do it if possible!
I don't think so, my closest mains cold is downstairs in the kitchen so would have to get a verrrrrrry long hose to get it from there to the upstairs bathroom tap. Kitchen is mixer tap as well but guess wouldn't matter if I just put on cold?
Thought wet vac was supposed to do same thing as this...
Vented system.
I was installing a new sink at the weekend and had to drain our cold water storage tank and hot water cylinder to be able to work with the pipes as no service valves previously and gate valves from CWST have all seized.
When finished I turned mains back on and tanks refilled but...