All good points gents and thank you for your feed back.
It could well be that I am tensed up and perhaps not aware that I am.
To answer all of the questions; I am 6' 1" and 13.5 stone and I would say average build. On both occasions I was shooting on a sporting layout. I am 47 years old and I had my first shotgun at the age of 11 and have been shooting rifles, handguns (before the ban) and shotguns all my life and a regular basis. I would consider to be fairly experienced but I haven't (I don't think, experienced this headache thing before). The gun is a Browning 525 Sporter Adjustable 30" and has been fitted correctly. I don't get any issues with pain on my cheek area in terms of my cheek weld and associated recoil and also no bruising or shoulder pain either - at all. I was shooting with my gun mounted (i'm thinking this could also perhaps be a contributing factor rather than starting unmounted with perhaps a more natural relaxed position). I don't feel I'm being knocked about by the gun at all and recoil seems/feels minimal. It's a lovely shooting/fitting gun and seems to hardly have any felt recoil. Actually on that last point - that is the strange thing. I feel that I could keep shooting and shooting it is that comfortable to shoot with which is the bizarre thing. All shots are taken in fairly quick succession - probably 10-15 in pretty quick order and then onto the next stand and so on.
I think I've answered all the questions and very good to get everyone's opinions and also very interesting to know that some of you, if not all of you think that gun headache is a real phenomenon. I am going to the same clay ground this weekend again and I will certainly check my stance and shooting position - I do think the relaxed point is worthy of investigation.