The ATPL's (up to amendment 2 or 3?) count indefinitely towards the issue of an ATPL(H) when the remainder of the experience requirements are met (IR for ATPL IR, night hours, multi-pilot hours etc) however if you don't get the IR within the 36 months you have to resit at least 4 exams in order to have the IR theoretical knowledge current prior to undertaking the IR practical course.
There is also a caveat about having maintained a current type on your CPL or something... the wording of which doesn't spring to mind.