It would be reasonable for me to assume...

[itsobviousright]

that a man in his 50s, who normally would socialise once or twice a month with friends, but is now having 12+ social engagements within 6 weeks, 3 of which include overnight stays, with a further weekend away planned in the new year...then these social occassions aren't with friends, but another woman?

Suspicion further fueled by spotting messages (innocent but with kisses) popping up on his phone from a woman who's name has never been mentioned (regularly talks about all the woman at work otherwise)

Currently in a separated but living together situation, so another woman isn't so much of an issue, but I'm surprised at the ferocity of my feelings around this - possibly more fuelled from the fact that he is spouting an incredible amount of bullshit/creating made up plans to cover all these evenings and overnight stays. I'd much rather he was honest!

And yes, I know I need to leave, but large amount of debt, difficult market for house sales, other tricky family issues etc have stalled things considerably

It does sound likely but also nothing to do with you

That’s a more than reasonable assumption. BUT if you aren’t together then technically not wrong (bit quick though…) and he probably is being secretive so as not to upset or anger you or make the living situation worse.
Maybe be blunt and say you know he’s seeing someone, stop the excuses and just be honest to make the living situation less uncomfortable with constant egg shells.
But debt aside, you need to extricate yourself from this arrangement sooner rather than later. Of course it makes you angry, you’re legally married and living together. Even if you are separated, as long as you are living as a unit of sorts you will feel like you are being cheated on. And that will be so harmful to your mental health. And it doesn’t leave you in a position to move forward in your life and get over the person you still see everyday. You need space to grieve. Everything else is secondary.

Currently in a separated but living together situation, so another woman isn't so much of an issue

"So much of an issue"? Since you are separated it isn't an issue at all. It's understandable that you are upset by this, but your marriage is over and he is moving on.

The dismantling of a marriage is a painful process and living under the same roof is untenable for most people. Move heaven and earth to get out of this situation. Time to focus on yourself and healing so you can move on as well.

Sorry to be so blunt but there's no point in sugar-coating the situation. Divorce is bad enough without having a front row seat to your ex's dating life!

It sounds like you really need to separate properly. He's obviously having a nice time seeing someone else and it's time for both of you to move on. If he keeps up with his ridiculous cover stories, I would just say 'You're obviously seeing someone so please spare me the bullshit'

I do know how you feel because I am currently also living with my ex and it is not an easy thing to do. Have you discussed it at all when you were separating? We did and agreed that until I move out, neither of us will be dating. And if we do happen to meet someone, then we will be honest with each other. We were always good communicators so it seemed like an obvious choice for us to set some rules around separation and living together.
To be perfectly fair, I am not sure you would necessarily feel better even if you knew the whole picture. Separation sucks and seeing another person moving on sucks even more. Yes, technically, it is none of your business but it doesn't mean you can just switch off your feelings.