On the bath only not the basin.

Leak? Or faulty tap?

I’ve got that too!

Race you to an answer!

If some kind soul could report bike forumage as per usual that would be great.

Bits inside the valve bit reducing flow?
Take it apart and have a look.
If it was a leak, there would be a leak.

has it always been like this or just started being like this? I had a tap with poor flow – it was a bit of debris in the pipe from when it was plumbed in. Other option is a failing valve

What type of tap is it ?
Ceramic cartridge quarter turn or tradition multiturn ?

A gold one. 😀

Multiturn.

I’ll attack it with tools tomorrow.

I hope it means i have to replace the whole chuffing bathroom.

Anything to get out of decorating

Take the top of the tap off and check the washer.
Check for any isolation valves down stream that might have been left in a partially closed position.

does it have a tap flow regulator? are you in a hard water area
remove regulator and clean/replace as required

A gold one.

forgot to say……..this is clearly your problem !