Saw a tiny bit of coolant on the grill when cleaning the car the other day. Then yesterday saw a little under bonnet steam. Sure enough there is the smallest of leaks in the top of the rad (about an inch from the top hose inlet). I was miles away from home and chucked a can of radweld in. It hadn't stopped it yesterday but will have a look again today. With hindsight I probably could have got home with it the way it was and as I can get to the inside of the leak from the top hose hole some sort of gunk could be applied by hand.
The radweld is in there now anyway, not much I can do about that other than flush it.