Thanks all.
"This friend who is currently acting as a go-between is totally kosher yes ?"Yes, 100%. She doesn't work at DH's school. She is currently unemployed having been made redundant during job cuts at county hall. She was in SS. She doesn't/didn't know what had happened - but she had had a chance to find out how I was feeling so DH was I think, mentally taking a breath to see what his revelations would do to me.
Anyway, I think he was 100% honest. You wouldn't say what he did if you weren't being.
He loves her. THey have been having a 'relationship' since about Jan. THey have kissed. They have not had sex. She ended it last monday because she knew she 'couldn't have all of him'. I beleive him FWIW. Please please don't all pile in to tell me he's lying because I don't think he is. ANyway as I said, the sex wasn't the issue. He had already admitted to the greatest betrayal.
SOme of his friends guessed something was up because of all the texting when he was out with them. He told one friend he was getting close to someone at work. Nobody at the school 'knew' but I suspect that lots of them guessed.
The only thing that confuses me is how the other teacher knew that they had 'broken up' - since no-one else should be privy to that little nugget 
I am having suspicions about ow - maybe she had been spreading rumours herself.
Am numb. I shouted and cried and said horrible things about both of them at first. The I apologised for being rude about her because I'm nice . I actually wanted to laugh - it really was male mid-life crisis bingo! Even DH admitted it.
However hs is very sorry, remorseful and guilty. He won't allow me to blame myself in any way - I tried to make it my fault as I tend to do because I was depressed and tired all the time, but he wouldn't hear of it. It is 100% his fault. He loves me. It was just that he also loved her 
He has agreed that he won't text. After next week they won't be working together at all and he won't spend time with her. I have also said that he won't be going to any school social events, at least not without me and he has agreed.
Not sure where we go from here. Suspect I will start ranting soon. But right now I am calm. I have to weight up ending a long relationship with huge amounts of shared history that has been normally happy and contented, with a broken marriage and 3 devestated kids. I can tell you now that DS1 who's relationship with DH is a bit rocky at times, would never ever forgive him if we split up because of the ow.
My self-esteem is a bit rocky right now but TBH DH has always told me I am beautiful and clever and downright wonderful. I just stopped hearing it.
Please, please don't tell me I shouldn't beleive him and that I should dump his sorry hide. Because I need time to think about what should happen.
BTW I won't text ow because amazingly I still like her and feel sorry for her in her shitty little life. Maybe I'm not normal