I have now managed to find out some more details, none of which make this any easier to deal with. It turns out it was an escort he visited AT HER HOME and that he received not only a massage with happy ending but bl*w job as well. Although he had been out drinking with friends earlier in the evening, he went to this place, by taxi, alone and wasn't wasted by any means. Pretty calculated if you ask me.
He maintains that this is the first time he has ever cheated and that they didnt have sex, although I dont think I can believe a word he is saying given that he has categorically lied to my face about things until he has had to come clean when I produce evidence (Uber receipt to random scuzzy area of London etc).
In a weird way, I take comfort from the fact that this seems to just be about sex as opposed to him having some emotional connection with someone else, but then think to myself why the hell would he potentially jeopardise his marriage, home and full time access to his two gorgeous babies for some stupid sexual thing?
Aside from the hurt, anger and disgust I'm feeling about the actual act, I think I'm actually more upset that the trust is now gone and that things will never really be as they were before. Prior to this happening, I (and I think lots of my friends) would have considered my husband the absolute LAST person to cheat (let alone with a prostitute) - he is literally the most kind, funny guy and up to this point has been a fantastic husband and father, which makes this bolt from the blue discovery even more difficult to deal with.
I dont think in my heart that I'll leave him for this though. I know that sounds weak in some ways, but I dont think its fair to deny our children their dad because he couldn't keep his d*ck in his pants and I do still really love him even though I have lost trust and respect.
Sorry again for the rambling (and swearing) - I am decidedly more angry today! I have just contacted a counselor though, so hopefully we can find some way through this. Thank you so much for everyone's replies - they have really helped in this truly cr*p time!