The exemptions are issued to licence holders applying for an IMC on the basis of an ICAO IR(A) or military green Instrument Rating (Aeroplanes) and have passed a single-pilot IR test in the 24 months preceding the date of an application for an IMC Rating. The exemption is issued as they are exempt from taking the initial IMC Flight Test and written examination.
Note that it says Initial Flight Test. It makes no mention of subsequent tests or any reference to the requirement to hold a valid C of T.
My belief is that to be legal the C of T for the IMC should be signed by the TRE who signed the IR then you have 25 months.