Who should convene which type of meeting should be set out in your standing orders and as a general rule, extraordinary meetings are convened by either the Chair or by two or more councillors (as specified in your SO).
The definition of an extraordinary meeting can vary. If a lease/deed is to be signed, then SO usually indicate that an extraordinary meeting is required which should set out that a lease/deed is to be signed but its arguable that a preliminary discussion on a draft lease for example might be considered "ordinary" business so the detail might be relevant here. It is also relevant that from what you have said, the chair would appear to have an "interest" in the subject matter so may feel it inappropriate to sign the agenda.
There is a general premise in meetings law and procedure that any decisions taken at any meeting that is convened in accordance with the requisite notice, publication, (i.e. 3 clear days notice sent to all councillors and displayed on notice boards and website) and is quorate are not invalidated on the grounds of a minor defect in the way the meeting is convened such as the wrong person signing the agenda.