From earlier posts on this topic it appears that a 'LockShield Valve'
or a 'Radiator Valve' can be used to balance DCH flow.

Apart from about 4 quid, the obvious difference to me is that the
radiator connection on the LSV is external by means of spanner, and the
RV is internal by allen key and the LSV appears to have a crush type
copper washer to seal the valve instead of a spherical seating.

One benefit of the LSV is that it's possible to use 2 opposing spanners
to prevent the threaded radiator insert from rotating when tightening
the valve - whereas with a RV there's a risk of the insert moving and
accidentally bending the 15mm copper pipe.

I've pretty much answered my own query but would appreciate any
comments, particularly on the copper crush washer - i.e. are they one
shot items that are replaced if the rad is removed?

If anyone is thinking of replacing please note that the centres appear
to be the same as TRV at about 45mm from the end of the rad, instead of
35mm for a RV.