Hi,
I have been through exactly this process last year (bought a house built in 1950s, from owners in the 90s, who had not registered the house with Land Reg and had lost the original deeds).
It was a stressful process, but we got there in the end and did not delay things by too much. Start to finish from offer to move was 5 months.
Vendors went through of process of demonstrating their ownership of the house through sworn statement and by supplying evidence of ownership e.g. council tax records for 15 years.
House was then registered with Land Reg using an expedited process (turnaround was 10 days) - conveyancers can apply for expedited process where it is holding up a house sale/chain. No issues being accepted for expedition.
House was registered and sold to us with limited /presumed title - this is my lay person understanding! I believe this means there is some remaining risk that the vendors duped us and the "real" owners with the title deeds are out there. That needs to be legally accounted for, even though the risk is remote. If these "real" owners exist, they can claim the house within ten years of our purchase date. We were given an indemnity policy as part of the sale to cover us if this comes to pass. But obviously based on the evidence they've had to supply to get to this stage, it's HIGHLY unlikely. It's worrying, but you have to really think through - is that a likely scenario given what we know about this sale? It seemed very unlikely that two 90+ yr olds are falsely claiming ownership of a house that they'd raised their children in since the 1950s.
Once ten years pass, we can apply to land reg to re-register us with title absolute. Everything reverts to normal.
If we sell within ten years, the clock does not restart for the next owners, they can apply once ten years from our purchase date. We do not intend to sell in the next ten years so this is not a huge concern for us.
Hope that helps! Our conveyancer said it's uncommon but certainly not unheard of and will soon die out as an issue now all houses need to be added to land reg.