Hi, In the end I replaced the PRV and that seemed to solve the problem. It seemed that it was clogged up with black gunk (which was also the same problem with the pressure gauge) and this was preventing it from functioning correctly. After replacing the PRV, all was good. I treated the system with a can of the sentinel stuff which you pump straight into the radiator bleed valve and everything has been fine since. I still have to top up the boiler about once a month, but no clunking and it has been performing as it should through the cold winter! Hope you get yours sorted.