Thanks for all your help and advice. Since last night I've had the valve on the downstairs loo radiator almost closed. So far, it's heating up fine (and I can even have it at setting 3!) and I've noticed a difference in the garage radiators already. I'll see how things go before getting him...
I recently had a plumber install an additional 2 radiators (both 700x400) into my gas central heating system. Forgive me, but I'm not altogether technically minded, but I know that he took the supply for the new radiators from the pipes which supply a radiator (400x300) in the downstairs loo...