Does your lease address a period of time for the company to act on the expiration? That you've entered into negotiations might also influence the termination. Careful from here on out, and a lawyer might not be a bad thing to have behind you.
Your lease expired by its own terms and you don't technically have to do anything at this point... however unless you are dealing with the same company that leased you the first time around then another company might want to see the first lessee file a release of the lease of record. If you're dealing with the same company...it becomes a moot point.Alternatively, all you need to do is file a very simple Affidavit of Non Development which affirms that there was nothing done on the lease premises, or on lands pooled therewith that would have served to perpetuate your lease. This is the second best route.