So all my radiators in the house have thermostatic valves apart from one (which is a different style of radiator so I think it was probably fitted at a later date)

Looking at the photo I think the thermostatic head should go on the right (I think this is the return flow?)

and I’m looking to buy just the head

They all seem to say they fit M30 x 1.5mm threads. (Eg https://www.screwfix.com/p/drayton-rt212-white-trv-sensing-head/569hg) I’ve measured everything on the right valve and can’t see anything that measures that size, but it also says that’s standard

What am I doing wrong?

All my radiators in the house have thermostatic valves apart from one (which is a different style of radiator so I think it was probably fitted at a later date)

Looking at the photo I think the thermostatic head should go on the right (I think this is the return flow?)

and I’m looking to buy just the head.

They all seem to say they fit M30 x 1.5mm threads. (For example) I’ve measured everything on the right valve and can’t see anything that measures that size, but it also says that’s standard

What am I doing wrong?