renew my ATPL to keep the imbedded IMC rating
UK CPLs/ATPLs had imbedded IMC Privileges (
not Ratings)
UK Issued JAR CPLs/ATPLs did not
I still have a valid IR and assumed that my new PPL would have an automatic IMC rating
You've never had an IMC Rating. Why would you assume the CAA would issue you with one if you didn't apply (and pay) for one?
Apply for one now - on the basis of your last IR Test in an aircraft, if that was less than 24 months ago. Otherwise take the test and apply.