I have an estimated Deed of Rectification acceptance date of 26 May 2026 - is this really corr?

I am selling my flat and during conveyancing we have discovered the updated floor plan for my flat, drawn up in 1991 at the time of my purchase, was never formally registered with Land Registry.  We have submitted an expedited Deed of Rectification to process this 34 year old plan but from an online check on the progress of the application, the estimated completion date is given as 21 May 2026.  The buyers solictors will not proceed without the acceptance of this plan.  Does this really mean I must wait 15 months for this?  

Simon - the estimate is correct but largely irrelevant as you are selling. If you have a confirmed buyer then your conveyancer/you need to request expedition - Request an expedite - GOV.UK
If they/you have done that then once expedited it will generally be considered within 10 days. The confirmation re expedition explains.
Everything then hinges on the application being in order of course. Hopefully your solicitor can confirm that it is.