Hi, yes I have read through that previous post however my circumstances are almost opposite.
He is in a very stable environment, which has not changed at all since we got him. Same family members, same daily routine which includes two walks which always include some intense exercise for him. (We are the only owners he has had)
When he first bit it was one of my sisters friends. He had made it known for a good while beforehand that he was upset and was wanting to be left alone (he was growling and trying to avoid them) and he was separated from them. However they went back to him despite being warned, and one of them went down onto the floor with him and was face to face with him, which was when the bite occurred (I'm not trying to defend him biting but just stating all the facts). As punishment I told him off verbally and took him through to another room and ignored any attention he tried to get for a good hour or so, he definitely knew he had done wrong, as he was very sheepish and quiet for quite a while afterwards. Due to the circumstances of the bite, after keeping him on a tight lead for a few weeks in which he was entirely his normal self, I began becoming more relaxed with him again. This includes seeing regular faces on walks off the lead and him mixing with big crowds in pub etc. which he always loves. He will make a point of going to say hello to everyone he can tail wagging and perfectly happy.
However this time was, as far as we know, completely out of the blue. He was laying on the floor next to someone, she lent down to stroke him and he jumped and went to bite her. Now like I said he had spent the last few days with this person, and had displayed no warning signs beforehand. The only thing linking both the incidents is they occurred at home, and have both happened to friends not those he lives with. He is not a particularly territorial dog, he will have the odd bark when someone knocks at the door but as soon as he sees its a person he is always tail wagging and wanting to get a stroke off of them. He is also used to having his mouth be handled when having his teeth brushed etc and he has never once closed his jaws on my while I have been doing that.