Just wanted to say thanks guys for all your excellent advice. I had to replace a leaking TRV and everything went fine, but so some reason the bedroom radiator stopped heating up, I noticed that the radiator that had the leaking TRV was now getting a lot hotter. So I followed the tip on here about how to quickly balance the system, 1 turn for large and 1/2 turn for small and the difference is immediate. All radiators heating up. I will probably do a balance using a temperature gauge when I get one, but this method has worked fine.