So are you saying your cold supply is from a tank in the loft too?
Logically, if so and the tank is full of water, and you can see the exit of the cold water pipe in the tank is clear, then you have a blockage in the pipe from the tank to your bathroom. Something fell in the tank and is stuck in your cold water pipe to the bathroom. Wouldn't like to speculate what...
Not sure how you'd shift it either. If you could blow pressure back up the pipe maybe that would blow the blockage back into the tank. I can think of ways to blow pressurised gas back up through the bathroom cold pipe but I'll not suggest them here for fear of attracting criticism ;0)
Ditto with doc martin, Personally if you can't see an obvious a blockage i would drain the loft tank & find someway to run a hose off your kitchen or outside mains tap & connect to the slow bathroom tap & see if you can use the mains pressure to push the blockage back up the pipe into the loft tank, but make sure you have someone in the loft to see if anything comes out the pipe & also not filling the tank up to much.
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