I posted, BigBadMummy, because I was looking for some reassurance. I probably wanted posters to say "Oh don't worry, he may have looked at the site out of interest, but it no way means he has done anything physical with some one else." Which in fact is what I believe. I was quite shocked at the time, and wanted to tell someone about it, but couldn't say anything to anyone in RL. Which is the beauty on Mumsnet.
At the time of posting the OP, I didn't know if I was going to confront DH or not...I was kind of working my way though my feelings. I may bring it up in the future, I may not.
I probably won't to be honest. If he was shagging around he would deny it (if he valued our marriage) and if he didn't deny it, we wouldn't split up, so effectively I would be condoning his infidelity.
Maybe I'd rather be one of those old fashioned wives that like to turn a blind eye, or maybe searching his PC history was a bit like looking into his mind when an attractive woman walks by...I really don't want to know what he's thinking, but he's perfectly entitled to those thoughts, as long as he doesn't act on them.
Having given the situation some thought, for several reasons, (which I won't go into), I am actually quite confident DH was just having a curious peek.