No provision of this Measure or of any instrument made under Part II or Schedule 1 shall adversely affect the tenure of office or any right to pension of any person who, immediately before the relevant date or, in the case of an instrument made under Part II, the coming into operation of the instrument, holds or has held a freehold or other office conferring fixity of tenure in any cathedral unless, by an instrument in writing under his hand, he agrees to be bound by that provision.