Property Deeds

My sister & I are attempting to sell the property of our late Mother. It isn't registered. Despite all the deeds our late Mothers solicitor held being sent to the firm handling the conveyancing, they claim there is a deed missing. a deed raised in 1956 when the house was bought as a new build. Could anyone please help or give guidance where to start in finding this paperwork ? Best regards, Jon Pollard

Jon - if it's unregistered then the family, home, solicitor and bank are the only real places to search unless your late Mother had an alternative 'safe place'.
If you have everything else though that seems unlikely and it reads as if that one deed has been mislaid. 
The conveyancer will need to consider applying to register it based on 'lost deeds' but do be guided by them - First registration of title if deeds are lost or destroyed (PG2) - GOV.UK