He can apply now after 12 years and obtain was is termed a ‘possessory title’ and then after another 10 or 12 years, can’t remember which off the top of my head, you can apply for the title to be upgraded to ‘absolute title’ (best class of title)assuming no-one has queried your occupancy in the intervinig period. I’d get your seller to apply now for possessory title, at his cost and hassle – if the Land Registry kick it back you’ll know where you stand.