Hello Gary - a deed of rectification would only apply in a very specific set of circumstances -
Rectification and indemnity (PG39) - GOV.UK
I assume the flats are both leasehold so you need to be looking at the two leases as well as the title plan. And if the flats are mortgaged then the lenders also need to be involved as appropriate.
If an application is required to amend the registered information, be that a deed of rectification or say a deed of variation then it would need to be submitted and then expedited to ensure it is processed as swiftly as possible -
Request an expedite - GOV.UK
However it the leases are correct and the registered information is wrong then you should report it as an error -
Public guidance: dealing with land and property - GOV.UK