Thanks all for information and suggestions. In an effort to bring things to a head I once again took to the road today. Before starting I checked if any loss from the radiator had occured overnight; negative, and again no contaminated oil. I drove the car at a steady 30/35 mph for 15 miles, stopped and checked the radiator when I saw a small whisp of steam from the bonet; the level was 1/16in above bottom of header tank; also checked exhaust for steam, but quite clear. Topped up radiator to 1in from cap and returned home (15 miles), on arrival no water in header tank! But one clue, a small white line has appeared from the front core plug, could this be where the water is leaking from? Presumably the heat of the engine would instantly dry up any leaks here. Is this a possibility? Incidentally the head gasket is new and I don't think this is a problem; also the radiator overflow shows no sign of discharge.
Sorry for 'going on.'
Peter

It’s worth a mention here, that although it is the opposite to your problem to regularly check that the overflow pipe is clear. I use an old cycle brake cable inner with the end splayed out. It is surprising how much grunge collects in the small bore of the pipe. If you are unlucky and you have a well fitting cap you could ruin a good rad with the pressure build up.

Peter, if a core plus leaks it tends to be messy. no instant dry . i'd cross it off the list. Mac