Thank-you all for your responses. And apologies for the apparent drip-feeding. I have been suspicious for a while, but have been waiting for proof, and wasn't sure if this was it. And being constantly told I am unreasonable to be uncomfortable with this relationship hasn't helped with seeing the wood for the trees.
To answer some of the points above, this was a planned weekend away, for a specific music event, which was happening on the Sunday. They were due to be staying at our holiday house, which my husband has been doing up. He assured me, as ever, that he would be sleeping downstairs on the sofa. Before he left he checked that I was OK with him going. At this point I was.
He left on Saturday mid-morning, as he was also needing to a bit of work on the house. But the photos also show them having a lovely day out, so .
I didn't hear from him till a few texts in the evening, usual are you and kids ok type of thing. I texted first. Sunday morning he calls, and casually mentions, when I asked what he/they got up to the previous day, that she had, as a birthday surprise, arranged for him to have a massage at a local spa. He even said we would have to go there when we were next at the holiday house.
So they do their music thing on Sunday, back late so don't hear anything till Monday morning, he is hungover, returns later that day.
Evening and overnight he begins to feel unwell, virusy type thing, so takes himself off to bed early. He is also up very early and goes downstairs (this morning). Then comes up and brings me a tea, and goes back to bed and sleep, as is feeling really unwell. I think as a result of this, he does not realise he has left his iPad downstairs and logged in to his FB. Normally he doesn't let it out of his sight, and it is pass code locked, so not sure why it stayed open this time. But when I went down for breakfast, I saw it there, and thought it was too good an opportunity to miss, to check out what was happening. The first thing I saw was a message from her to say that the photos were up, but only visible to them both. So I had a look. So it seems that the Spa thing involved robes, spa bath, as previously mentioned, also dinner with bubbly - not sure who paid for this bit, and lots of photos of the meal and food , and of their day out. The message also said thanks for the weekend - lovely as ever x
He always minimises, says they are just friends, gives me the barest of details. I need to get my thoughts straight, of how and when to confront.
LTB is as ever easier said than done - we have a 16 yr old DS with ASD, in the middle of GCSE exams, and a 5 yr old DD, who worships her Daddy .
We're meant to be having a lovely weekend away as a family this weekend, for his birthday, and I have cooked him his favourite meal to take with us.
Sorry about the length of this post - filling in the gaps. Does help to get it down.