They’ve offered you a mortgage haven’t they? So hopefully all can still proceed for you. Have you got to wait for your name to come off the joint mortgage to proceed?
His solicitor may well be advising him exactly that but it doesn’t mean he’s listening!
I can’t say if you can or can’t force a sale (I would doubt it as you’ve had your lump sum and his solicitor has advised they are dealing with things) but if I were in your shoes I would rather stick with it and let him buy you out. Forcing a sale through court will take months, then you need to agree on a whole range of things (sale price, estate agent, conveyancer) then find a buyer, agree a price, be in a chain etc etc Trust me, selling a house with an ex is beyond frustrating. Him buying you out is what he wants and therefore he will be cooperative, forcing a sale will irritate the situation further and will definitely take much longer.