On 2020-05-26 10:41:59 +0000, John Rumm said:

i presume the system needs a drain down to do the valve but it's just a
couple of large screw nuts.

Yup in principle - although it does depend where in the system the
valve is, as to how much of a drain it will need (valves in a loft for
example can often be change with only a partial drain down).

Thanks John. What do you think of this - it's the same Danfoss valve
and as the pipework is at the back under the stairs it may be easier
just to change the spindle but looks hairy without draining down. I
guess easy to do if drained down and guards against disturbing pipes as
the cylinder is very near.

https://youtu.be/1vYCkGtR-l8