1st thing to try is draining the system and then carefully refilling to rid of any possible airlocks. turn off stopcocks (prob in roofspace), drain system via taps and then turn on stopcocks slowly with taps still on to let any air be worked out as the system refills. this worked for my...
i had similar problem - then figured out that the metal hinges on those wooden loo seats can normally be rotated - freeing up more angle for the seat to sit up. fixed my problem.
