oh wrote:

My En-suite basin cold tap does this also. It is because both hot and
cold pipes are close together under the floor so the heat from the hot
pipe heats up a short section of cold pipe and therefore the water
inside.

I was thinking that at first but it doesn't matter which tap I run and
it happens even when the central heating is off and I haven't ran any
hot water taps.

But assuming your HW is hot, there will still be at least a hot pipe
from the boiler feeding the HW tank, no? So all that has to happen is
for this pipe to be in close proximity to the cold water pipe somewhere
along its length.

David