Our downstairs loo hot water tap knocks and bangs when turned on slowly, its fine when turned on fully.

Is it air in the system, water pressure or something else? Easy fix?

http://singletrackworld.com/forum/topic/regular-knockthud-in-house

some knowledgeable people on here talk about this exact phenomenon

‘water hammer’ apparently

Ours does the opposite. If half on, it’s okay, but full on it makes a constant whine but doesn’t knock. We do have really good water pressure though.

I think turning the stopcock down might solve it but I do like a powerful shower.

If you only get the noise when using this particular tap, it most likely needs servicing or repair. If it is a washer type tap, knocking can happen when the washer becomes detached from the jumper or the jumper itself is loose in both cases the water will cause vibration in the tap as it passes through the water ways.
It’s an easy job to check. Isolate the water supply. Hopefully there’s a ball-o-fix valve under the basin. Remove the tap handle and possibly cover. (this could be one and the same.) Then remove the tap head or cartridge. Examine and repair as needed.
Assemble in reverse order.
Use this for reference.

Had this once and had a somene look at it. He said told my wife it was caused by way the house was plumbed. At this point I would have preferred for her to point out that if that were true why did it start 3 weeks ago and has never been heard in the previous 10 years of living there rather than paying the call out charge (later refunded).

Had another plumber come and look – ball valve in the header tank had gone -£60 ish from memory.

Turned the water off last week and back on yesterday. I did the standard internet water air purge routine but I need to tweak mine; due to the odd plumbing it won’t work if I include the ‘lowest’ tap- the outside tap. OP try the standard purge then if it doesn’t work try excluding a tap.

Edit- your issue make be a dicky tap- self replace isn’t too hard. Or plumbing to that particular tap has a dead end?