Don't get me wrong Bclam, I wasn't criticising. A UK CPL (and by implication an ATPL) includes an IMC rating. So long as the licence is current you have IMC rating privileges without any need for a proficiency check.
A JAA professional licence does not include the IMC rating privileges. You can add the IMC rating to a JAA licence and you then have to maintain it in the same way that a PPL holder does, by periodic proficiency check. The only easement is that if you revalidate a single pilot Instrument rating that automatically revalidates the IMC rating.
The bit that Aerotowman was referring to was the fact that an Instrument rating revalidated on a multi-pilot aeroplane is not valid on a single-pilot aeroplane and vice versa. That is true no matter what licence you hold.
Since you have a UK ATPL(A) you automatically have the privileges of an IMC rating so long as that licence is valid, as listed by BlueLine above.