Dear All, thank you so much for your comments, they have made interesting and thought provoking reading. The consensus appears to be stop the GF visits if they don't abide by the house rules.
However, and apologies for not providing full details in first message, there is a further dimension which complicates this. DS stays in the school hostel on the Mainland, during school days so GF and DS see each other very frequently during the week and evenings too. DS's mother lives on same isle as DS's GF, and as GF lives quite close by she is a regular visitor and is allowed in DS's bedroom when he/they stay there. I don't know why GF needs to stay overnight at DS Mum's place as she lives close by, but when she does she has to sleep downstairs, nevertheless I understand that there are hours during the day and evening when they are in the bedroom with door closed. Public signs of physical affection appears not to be a problem at Mum's house.
I only see DS on alternate weekends, so I am concerned that if the rules here are too strict and, as suggested, I say that GF visits will stop unless house rules are observed, then not only will visits form GF cease but also from DS too. He will possibly/likely stay at his Mum's house where the rules (being zero) are more to his liking. I have tried to discuss this behaviour with his mum, but she doesn't see a problem with two teenagers spending hours on end unsupervised in a closed door bedroom and public signs of affection are ok, "they're only kids....." . I guess most, like I, will disagree with DS's mum. But I can't, and have been told so, make rules in her house. I wish to maintain contact with my son, but it is becoming quite difficult witnessing this behaviour which makes me uncomfortable in my own home, but when challenged he becomes quite angry and almost aggressive in his stance and manner. Just to be clear its not just the occasional hug and kiss, it is public, almost constant and they are hardly ever unlocked.
I have spoken to them both again and wait to see if they change their behaviour, but I have my doubts. I also intend to speak to GF's Mums, who doesn't seem too concerned.
Thanks again for your replies.