Replacing old thermostatic radiator valves

I have to replace 20 year old TRV's that have what appears to be about 20mm diameter fixing to the radiators.

I can only see narrower ones in the DIY shops.

Are there special TRV's to replace these or are there reducing fixtures that are to be used in the radiator to take the modern, smaller diameter fixings.

Thanks

Nivek

if they really are 3/4 tails then you need to buy some bushes. 3/4'' to 1/5''

Prestex belmont valves from PTS @ £20.00 each inc vat. trade price .......just turned down a job like that ..so I got the prices first you could try taking the old tails out of the rad but you`ll need a "spud" because the really old ones don`t have a hex inside and you`ll need heat too so maybe the paint will burn on the rad .....and they`ll be full of black sludge etc. guess why I turned it down