So, to provide an update. Because I wanted to know what type of person this woman is, I did an extensive investigation and found out some things which were a little disturbing. Plus some comments made by her online showed her to be a not nice person anyway. I agonised as to whether I should tell him what I had discovered or not.
We had an argument on Thursday because he put me on speaker phone while she was in his car, but didn't tell me until I had said quite a bit. He does not think that what he did was discourteous, refuses to apologise and doesn't understand that I am more upset that he doesn't care that I was upset than by the actual incident.
So, the situation was already strained when we had our next argument on Sunday. It ended up with me telling him what I had found out. I explained how I had found her and he asked her about the particular thing, which she denied and then he asked her about comments generally on Facebook and she denied having an account. He then told her the profile name and she still denied it. Unfortunately, it was only then that he asked me to show him, by which time the account had been deleted.
During all of this of course he was angry and making his usual threats to move out. I said that I think he should and we agreed that he would start looking with the intent to move in the new year.
He says that he has broken things off with her but I am not sure. He read out a message from her where she said some pretty nasty things about me, showing her true colours. But because she is still denying having a Facebook account, even though I have given him names of some of her friends, he says that he doesn't know who to believe, which I find very insulting and if he continues to see her after this, I won't be able to remain friends with him anyway.

You won't be able to remain friends period. Give him notice, get it done. This situation is not healthy for you. Whatever contact he has with this other woman is not your concern.

Agree!

Catlover,

I'm sorry to be blunt...but I cringed reading most of this thread. You need to let this guy go.

It sounds like you call him your best friend to justify being overly-involved in aka obsessed over his personal life. I do not go investigating the romantic interests of my best friend. That sounds creepy and unhealthy.

The reasons you gave to delay kicking this guy out (holidays etc) are not any reasons that Dr Harley would support.

Not only would Dr Harley tell you to have him move out ASAP (I am talking days - not weeks) but to go no contact with him. You are in love with him and it is clouding your judgement.

This is a bad situation that you need to get yourself out of pronto.

In the future when a man tells you he is not interested in you, do NOT become his "friend". This is code for "wasting your time" hoping he will come around.

You have wasted four years of your life pining over this guy when you could have used the time dating other suitable candidates.

Honestly, catlover, you are wasting your time with this. I would move him out now. There is no legitimate reason to wait. You are just prolonging the pain.

Thanks everyone,
It is interesting that I call him best friend because on reflection, I would say that I don't have one although I am most definitely his. I agree that we won't remain friends for long after he moves as he does have his own friends now and can get on with his life.
I have had a lot of fun over the past few years so don't feel I have missed out on anything and I know I am not in love with him as he meets only about one of my needs. It is probably the behaviour similar to my ex's that triggers my crazy reactions.
He has started looking for places to stay things are moving and it won't be for much longer.

You are in love with him so your judgment is clouded. It is not a good thing to waste years of your life over a man who does not feel the same way about you.

The reason I feel the need to point this out to you....is because if you continue to allow your feelings to lead you astray, you are going to continue wasting your time on this guy, even if he moves out.

Can you acknowledge that your judgment has become clouded or do you not see that?

Thanks SusieQ,
Yes my judgenent was definitely clouded in the beginning. I liked him, fancied him and he was my only friend living nearby. It was inappropriate of me to suggest friendship, especially knowing the future problems it would likely cause. I admit that before he moved in, I did have hopes that he would change his mind, so that would have been for about a year.
BUT, I have lived with him for three years and I am categorically not in love with him! I know what it feels like to be in love and, with him I am not. I do love him, I admit that but I also know that there are people out there that I can have a loving relationship with. It has only been recently that I have even felt like going out to events etc. I haven't not done anything that I would want to just because of his presence in the house.
I am looking forward to getting my house back, he can be a bit of a nightmare to live with.

You want the friendship to be worth something, because if it is not, you wasted 4 years. Like the emperor's clothes.