Thanks all for replies and perspectives
Some answers:
House is 2 1/2 bed 'snug', mortgage paid, it is both of our only residence, and was left as 'estate to be split equally', houses around here sell almost instantly (East Anglia - not too far from London)
I am aware that our house is also going up in value, however as I will still be buying the other half of a house, the cost of this is increasing if you see what I mean. Say all houses are 200k - we sell this, I buy an equivalent with my 100k and mortgage 100k. Say it goes up to 220k (double digit increases p/a not unusual in this area) I buy equivalent with my 160k, and mortgage 160k so it's cost me 10k (not including interest). That is significantly more than DB would pay in rent for that period, were he to do so.
I guess 10k perhaps not huge to some people, but makes the difference re: approved for a mortgage or not. My contract is also only another 18months, in one year when DB's contract ends I only have 6 months. I'd be faced with a bigger mortgage with less security, and I'd imagine I'd just be turned down by the bank. This is the situation that has me a bit stressed about speed of movement!
I did offer DB to stay rent free in my new house as long as he wished, I don't want to 'turf him out'. I can see how it looks as though I'm just thinking off myself, however, in life you do have to sometimes? He earns enough money, is planning to buy up north mortgage free, has no plans to ever settle down etc. We just have very different life plans, that's the only problem.
DP I suspect will lose the plot if faced with living with DB for a year in this house, it's a big ask - not that they don't get on, but it's not 'our' life if that makes sense. It would be an easier compromise, psychologically, for us to at least buy together and live with DB there, however I think that would make DB uncomfortable.
One idea I had was for DB to buy in the city he's moving to, let that out, and we split the rent he receives whilst he lives in my house- but he wasn't convinced.
I also suggested that I buy the house from DB, again allowing him to remain (for free - just bills) until his contract ends. But I think he thinks he'll lose out somehow if we do that (perhaps he would - however this would be post-improvements and I'd pay the fair market value)
I take the comments about spring selling and probate timescales , I have never bought/sold/inherited before so accept I don't fully understand the process.
What will be will be, I am prioritising my relationships with DB and DP over finances - but am in somewhat of a lose/lose situation on that front!