Are you using an up to date syllabus?, 'cos if you 're not somebody's missed a trick - the "exemptions" were documents issued by the CAA for "grandfather rights" microlights that hadn't met any airworthiness standard. The last of them were withdrawn about 1991 (possibly a little earlier, and they have no validity now.
The non-expiring permit to fly is what replaced it (and was always used for microlights designed around airworthiness requirements in the first place). It is very simply a document allowing an aeroplane to fly, which itself hasn't got an expiry date, but has to be validated annually by issue of a "Certificate of Validity" (a small piece of very thin paper reminiscent of a sheet of loo paper). The CofV is issued usually by the BMAA, after the aircraft's annual inspection and check-flight.
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