From the CAA LASORS document :
Differences Training in aeroplanes within the SEP class
rating is valid indefinitely. If a type, or variant of a type,
within the SEP class rating, has not been flown for
some time, pilots must use their judgement to decide if
refresher training is warranted. However, it is
recommended that such re-training be undertaken
when the lay-off is more than two years.
If a type or variant, within any other class or type rating,
has not been flown within the preceding two years,
further Differences Training, recorded again in the
pilots logbook or a proficiency check, on that type or
variant, is required.Instructors must consider the contents of Section F