Silly one, but you checked round the outside of the house for a dripping drain off valve? There often not where the boiler is, and I know a few folk that have torn their houses in two looking for leaks to find the drain valve outside at low level dripping into stones or soil
SGM means one of these,
http://www.toolstation.com/shop/Plumbing/d20/John+Guest+Speedfit/sd3307/Brass+Drain+Cock/p37798