Jamie - understood re one of the joint owners dying although you say 'we are selling' and you are not one of the joint owners?
If one of the joint owners has died then the restriction will bite and prevent a sale by the one surviving owner.
As such either the restriction needs to be cancelled or additional steps taken to enable the sale to complete
Cancel the form A restriction - see PG6 section 8 -
Practice guide 6: devolution on the death of a registered proprietor - GOV.UK
Enable sale to complete - see PG 21 section 6 -
Practice guide 21: using our forms for complex and more unusual transactions - GOV.UK