Hi Stuart,
There's a sealed cess pit behind the house into which empties all the black and grey water and I know they live in there but I don't think that's been compromised.  The only other grey water outlet is from the washing machine which empties onto the surface.  I realise there's a host of possible breeding places that I need to investigate by a process of elimination.  I just wondered if it was just me or whether it was general this year.  Yes, I do have compost heaps but they will be included in my search for sources.  Thanks both for the input.