What's this plumbing fitting

Isolation valve of some sort, I think. is it in the heating circuit?

Probably made by Valliant

This is close

It looks like a 15mm isolation (service) valve. Turn the knob to stop the water enabling you to change something up-line. If it hasn't been used in a good number of years it may be very stiff or seized. These type of valve are usually 1/4 turn off, 1/4 turn on and they will/can go the full 360 degrees (off/on/off/on).

It looks like a 15mm isolation (service) valve. Turn the knob to stop the water enabling you do change something up-line. If it hasn't been used in a good number of years it may be very stiff or seized. Thes etype of valve are usually 1/4 turn off, 1/4 turn on and thety will go the full360 degrees (off/on/off/on).

If it has an arrow on the body of the valve its probably a one way valve often fitted to outside taps to prevent syphoning dirty water back into the mains.