Thanks for your enquiry.
I don't see why the Mortgage Company require the details they have asked for, as upon the sale of the property, the mortgage would be paid off by your Solicitor?
I'm not sure therefore if the query is concerning the strip of land at the front?
If you could please clarify. When answering could you also confirm if they held the land at the front as Joint Tenants or Tenants in common?
You can certainly pay off the Mortgage before the sale completes, and if you do, the Mortgage Company shouldn't need any info from you concerning your Uncle.
However, if you do need his details 9which you shouldn't, you can try and obtain a copy of his death certificate using the following link-
Hi, there are Companies/tracing agents on the internet who should be able to locate the death certificate, but you would of course have to pay for this service.
If I have assisted, I would be grateful if you could rate my answer.