if one is based on the other then surely the dates must be the same.
Correct. See CAA Standards Document 14(A) para 9.8 for UK interpretation. Expiry date of your IR-SP-SE will be that of your existing IR upon which cross-crediting is based. When you complete your next IR, assuming you meet the experience requirements in Appendix 8 to Part-FCL you can then get your IR-SP-SE revalidated for a further twelve months.
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