In order to exercise your SP IR privileges it must be valid, this can be achieved by taking a revalidation test in a SPA. Once they are both valid, then you can retain the SP privileges by experience so long as the MP IR is also valid. Part FCL App 8
Provided that within the preceding 12 months the applicant has flown at least three IFR departures and approaches on an SP class or type of aeroplane in single pilot operations, or, for multi-engine non-high performance non-complex aeroplanes, the applicant has passed section 6 of the skill test for single-pilot non-high performance non-complex aeroplanes flown solely by reference to instruments in single-pilot operation.