Many thanks to all for such a large response - it is heartening to see how many opportunities there are out there to escape the city! I am moving to Greenwich, which makes Tring and the Striders a fair commute but potentially a realistic option for weekend running... It seems that Serpentine are the most convenient running club for where I will be living but they seem to be mostly about the Triathlon and the roads...?
Am following up all recommendations - so if any one has any further suggestions/anything to add (apart from stay North!), I will look into them...
Cheers!