Thanks for all your suggestions. Today I stripped down the shifter. The cables (including the brake cable) were in perfectly good order, no fraying or rust. I cleaned as far as possible the mechanism in the shifter and sprayed with GT85 (should I have greased it?). I also checked the cabling to the front mech, removed and cleaned the mech and replaced all components. There was very little change in the operation. The changing of the gear from the small ring to the large ring is OK. When changing to the small ring it does change, the mech moves OK, but to achieve this quite a bit of force is necessary on the lever and then suddenly there is a loud clunk and hey presto the lever moves and the mech changes position. Its like as if the shifter gets to a point and then suddenly falls off a cliff instead of a steady descent if that makes sense.
I suppose the bottom line is that it works. Not ideal for me and if I was using it as my main bike I would sort it, but my son in law will only be an occasional rider so I guess he will have to put up with it, unless he wants the LBS to have a look. For inf, although the bike hasn't been used much recently it has been fixed to my turbo trainer, inside my house with occasional use,and the 'fault' has been present for some time.