Then I just found I COULD NOT get him off.
Do not for one moment think that has anything to do with you. It could be any number of things from being too in his head thinking about what's going on, to nerves to having watched too much porn or having 'sorted himself out' a short while before you arrived.
I suddenly realised how weird and boring the weird little situation in this weird little flat was.
Totally understandable - it wasn't your place which can be uncomfortable as it is. The way things were going obviously didn't help.
he was .... kind of... not happy about this?
That was totally unforgivable and gave you valuable insight into his personality.
When I eventually did get up and get dressed he made no move to do any kind of friendly rounding off of the evening
Piss poor. No excuses for that!
I think he was really annoyed that I came and he didn't
That's totally on him and nothing to do with you. His response was wrong and, again, unforgivable.
Sounds like you just had a poor night with someone who was far from gentlemanly about proceedings Corvid. I hope you don't feel too bad about it because you certainly didn't do anything wrong.