As your IMC rating (IRR) is on the back of your licence then you will need to have it reissued but you are exempt from the IMCr training syllabus. It is my view that as your IR is still current then there is no need to do an IMCr renewal test which is required every 25 months. So see a examiner to renew your IMC which is a paperwork exercise and apply to the CAA for the IMCr to be re-issued - there will be a CAA charge.
However, if the IR is costly to renew because it is on a twin and that is the issue then renew it as a single engine IR. You will continue to revalidate every 12 months instead of every 25 months for the IMCr but the aircraft element will be at SEP costs. This way you will not be throwing away all the money and effort to obtain your IR. Use the IR revalidation test to also have the IMCr renewed. Then later if the IR was to lapse, and money is tight, you will have a further 13 months before the IMCr also lapses.