You may wish to remind the council of their obligations under the The Local Elections (Parishes and Communities) (England and Wales) Rules 2006, section 5(5), which states:
"(5) Subject to paragraph (6) below, where a casual vacancy in any such office is not required to be filled by election, the parish or community council must, as soon as practicable after the expiry of the period of 14 days referred to in paragraph (2)(c), co-opt a person to fill the vacancy."
Holding a vacancy for an unspecified potential purchaser of an unbuilt property is clearly not in the spirit of the legislation.