Hey guys,
Hopefully this clears it up.. straight from the horses mouth, Mary Stephans at CAA.
61.07.3(1) Candidates who obtain credit or a pass for the Airline Transport Pilot Licence (Aeroplane) (ATPL/A) subjects have 36 months to obtain an Instrument Rating (IR). The ATPL/A subjects will remain valid for a period of 60 months from the date of expiry of the last Instrument Flying Revalidation Check.
In short, you have 36 months to obtain your initial IF rating from date of completion of your last subject, thereafter your subjects remain valid for a period of 60 months, calculated from the expiry date of your last instrument revalidation test.
So in other words, they will never expire, provided you keep up your Instrument Rating.