Both licences are UK issued and have the same number. You can freely move revalidation pages from one to the other provided that the rating on the revalidation page is also in Part XII of the licence in which you want to use it.
The more astute will have worked out that the reason behind all this is the validity of my IMC flying using an IMC rating (in UK only of course) which my UK licence allows me to use if I have an instrument rating.
Wrong!
Your UK ATPL licence privileges include the privileges of a UK IMC rating, this has nothing whatsoever to do with the Instrument Rating or its validity.
Since Jan 2000 the IR privileges of a UK or JAA licence are exactly the same!
Most people keep their old UK professional licence simply to retain the IMC privileges.