I'm a 57 year old man, married 15 years to a 47 year old,  cohabited 5 years prior to marriage. 3 children - one aged 25, living independently, one aged 19 with an autism diagnosis living at home and not in work or education and one aged 17 at college. I earn £100K gross, my wife works part time for around £22K gross. House is worth around £450K, remaining mortgage of £100K. I have pension savings with a CETV of £400K, don't know hers but would guess less than 100K as she took time out with kids. I recently inherited £120K.

I understand that in a divorce my pension savings will be part of the overall pot and the house proceeds will be divided in her favour, potentially the whole amount. Does that sound about right? 

Second question is to confirm that it makes no sense to pay off the mortgage with the inheritance at this point? As I understand, that would make the inheritance a matrimonial asset whereas there may be some chance of me keeping some of it if it just sits in my bank account.

Third, my wife has what might politely be referred to as mental health issues and there is a non-negligible probability of a complete breakdown (up to and including stalking, harassment, suicide attempts) when I leave.  It is likely that my sons will end up living with me.  How would this affect the financial settlement, if at all?

During a period of separation I would plan to move out but continue to pay my share of bills (currently 75:25) and mortgage for the matrimonial home.  Does this seem reasonable?

1, Because of her lower earnings (and therefore mortgage capacity) your wife is likely to end up with a greater share of the equity in the matrimonial home than you but it would be a stretch to say she should get all of it. I think that is unlikely unless there is some sort of quid pro quo such as a clean break from her and/or a reduced claim upon your pension. You do also have housing needs.

2. You are correct about the inheritance and mortgage. If you used it to pay off the mortgage it would immediately become a matrimonial asset. So long as it is held in an account under your sole control there are good grounds for saying the inheritance is non-matrimonial. Keeping it separate also offers more flexibility.

3. If your sons live with your wife they add weight to her housing needs. Conversely they would add weight to yours if they lived with you. Mind you, I suspect a court is likely to regard the housing needs of a 17 year old as time limited.

4. You would be unwise to move out until the divorce and financial issues arising from that are finalised. There are all sorts of reasons for that.