Just had the central heating on for the 1st time this season, and have discovered a fairly significant leak (2-3 ltrs over 4 hours) on the return valve side of a towel rad which was actually shut off at the valves. Not easy to see exactly where it was leaking from but it stopped on opening the valves so i've left it for now for closer inspection at the weekend. Curious why it would leak only when apparently isolated?