UK ATPL and the IMC Rating
 

I am a retired pilot, still holding a valid UK ATPL (as well as a JAR ATPL), but have let my Instrument Rating lapse. I now plan to do some private flying. CAA flight crew licensing insist that before flying on instruments using my IMC Rating I will need to undergo an IMC Rating renewal test.

I am not sure they are right about this. I recall that all original UK commercial pilot licenses automatically conferred the privileges of the UK IMC Rating without requiring periodic renewal. Can anyone shed any light on this difference of opinion?

I believe that has always been the case. You get the rating issued on the basis of your UK CPL or ATPL (but not EASA), but it then needs to be validated as any other IMC rating i.e. a certificate of test in the last 25 months.
Article 68 of the ANO refers - CAP 393