Where can I buy a plastic turning cap for this?

[CluelessDIY]

The plastic turning cap for this valve(?) fell apart a long time ago - when we need to increase the pressure we have used pliers to open and close it, but it’s getting increasingly difficult to do this. Where can I buy a replacement cap please?

Try Wicks, they'll probably have it.

Screwfix / toolstation

Screw fix

Much better to go to your local hardware store. Show them the picture and they will be able advise you of the exactly right thing.

I can't make out the make.

Would a square spanner fit it?

If it was me, I would probably try to buy an identical valve of the same make, use the handle, and put the valve in my drawer of things that are sure to be useful one day. Often you get a red and a blue handle and you use one according to hot or cold water.

It seems to have a hose union but doesn't look like a garden hosepipe tap. What is it?

It's the valve we turn to up the pressure on the boiler - when we replace the boiler 7 years ago this wasn't replaced and the plastic white turning thing on top of it had fallen apart and was long gone.

Square spanner and make of valve are good ideas, thank you. We really only have a vague clue as you might have guessed. The engineer who serviced the boiler in August said he had a spare one hanging around and would drop it in but didn't.

measure the square, across the flats. Maybe it is a convenient size. I have a small old spanner for car brakes that fits a 3/8" and a 1/4" square, which you might think is outdated but sometimes fits things. If it is metric there will also be something to fit, perhaps intended for some other job.

A square spanner on a square shaft, if it fits well, will be better that pliers or moles, which will round off the square. Once it is rounded, it will get more and more damaged.

i use these Pegler valves, which are high quality and the "T" grips are metal, not plastic, but I don't know if it is the same fitting as yours.

Thank you - will try adjustable square spanner, makes sense that it would be less damaging than pliers.