Differences Training is listed in the ANO and requires an Instructor sign-off; the CAA provide FIs with stickers to place in your log book when they do it. For SEP aircraft they do not expire.
Familiarisation Training, is the application of common sense before you fly something you are not familiar with. It does not require a sticker or a sign off or an Instructor. It is up to the pilot to seek advice on how much training would be appropriate.
Incidentaly, Differences Training was born at the same time at the SEP rating! July 99.