Hi everyone - thank you for all your replies! Too many to reply to individually but will try to include as much as poss in this update:
I went for a walk this morning after telling DH that I'd learnt of the other story surrounding 'that night'.
Whilst I was out, I asked OW's DH to get the OW to contact me as I really do need to try and get to the bottom of this. While I was willing to try and move on when I heard DH's side of the story, it's horrible now to know that I really have been lied to.
I need to hear from her, woman to woman, her side of things. Whether I choose to believe her story over my DH's is yet to be decided - but I'm definitely someone that needs to know details in order to process the situation fully and properly move on.
When I got back from my walk, I told DH that the OW would be contacting me. He got mad. Annoyed that this situation was still going on and 'giving him grief', furious at the OW's DH for contacting me, mad at me for basically having not gotten over it yet. As he was flying off the handle, he blurted out 'ffs, next time I'll make it worthwhile'. I couldn't help but start to cry at that point.
I think it's ludicrous for him to expect me to fully be over it within 10 months. He doesn't get to say how long I should get to grieve for. He said he doesn't care what I do, or who I choose to believe, he's sick of hearing about it all.
I then just told him that I sincerely hope the next time he does it, it really is worth it and that I was 'sorry' that this situation is SO difficult and SO shit for him. Poor him. This must be so tough for him 

I've now taken myself back out to get my nails done and try and relax for a few hours.
Someone asked if the OW's DH has contacted me so he can see what my DH looks like - that's not the case as the two of them have met before, I've also met him too.
I'd love to be able to be someone that can just out this behind me, but ultimately I need facts and the cold hard truth. I'd rather be slapped with the harsh reality of what really happened than be constantly lied to.
I just really can't see why she'd tell her DH a worse version of the story if that isn't what actually happened. It makes no logical sense to drop yourself in it and say things that you didn't do. Ugh. I just don't know anymore.