Thanks for the reply, Ian. If you (or anyone else) had to guess, how would this method compare with just turning all the radiators off? Any advantages / disadvantages between the two?
Would be more beneficial to upgrade the controls to offer greater flexibility.
As to your question, it is more beneficial as it is as you are only turning 1 ancient valve as opposed to maybe 8 to 12 depending on the amount of radiators you have. The less valves that are touched, at that age, the less chances of a leak appearing.
I would consider a system drain, add in a 3 port valve, cyl stat and room stat and TRV's on the rads. Done in a day and you have a much more controllable system.
Jon