I'm touched and enriched by all of your comments and I will spend the next day or two reflecting on them. I do at this stage think it would be healthy for us not to live together... well, I can do it, but she can't. And if if was just us, we could part easily enough. But there are the children, and I just don't see why, if I am the one who wants the marriage to survive, and she is the one who has had the affair, that I should have to be the one to hit the high road. For that is her basic proposition.
And then, no doubt, the OM will in due course emerge from the woodwork, and take my place and attempt to parent my children. I'm not having it, and I feel very empowered by the comradely support in your posts above. Thank you all for your time and feeling in posting, I shall read and re-read with great care.