It was new chain time and I figured it was a good idea to change the freewheel as well. Leon Cycles sell the replacement 7 speed freewheel cheap enough but they really aren't great. I also wasn't particularly happy with the gearing, especially at the top end so I decided to put a Sunrace MFM4S (13-34) wide range 7 speed freewheel on as a replacement and I am really pleased with it. The jump between 1st and 2nd (10T) is quite big but I don't really use 1st unless I'm going up 15%+ climbs and in 48-13 I can happily get to 50km/h (30mph), where possible, without pedalling too madly. As @Sierratim will remind everyone - make sure you buy a freewheel as a replacement and not a cassette. And if you go for the MFM4S you will need a longer chain too - I bought a KMC 8 speed and took two links out of it.