When you say separated, is that legal separation? Have you begun divorce proceeding?
ifvthe answer is no to either of those, then in reality , in the england and wales, then the answer is yes he will have a claim to it as a jointly owned asset that came into your ownership whilst you were still legally married.
you may have some case if you were married for a very short time, no kids, or that you have lived apart for a long time.
it will also depend on the overall financial situation based on “fair settlement “ criteria the court must use when sealing any fincnail agreement including consent orders. These are 10 or so criteria that must be met before any 50:50 or discussion of agreements to not share inheritance
of course timings on probate may give you some degree of wiggle room, but I think if person has already passed away, probate is ongoing it’ll be considered a shame dunk you’ll get it and therefore will need to be considered in division and financial agreement, even if it’s not yet part of D81 assets you’ve already got.
it’ll come down in part to attitude of spouse - if they want half I think there’s little you can do if fair settlement terms have been met first,