Just re-read some of the earlier posts:

Time Immemorial does (or did) have a specific meaning in English law. In 1276, it was fixed by Statute that any custom or right which had been exercised since before 1189 (i.e. the end of the reign of Henry II and the accession of Richard I) should be regarded as having been in use since "time immemorial", and therefore did not need any specific grant to be exercised.

A requirement was that any such right or custom should be "continually exercised" i.e. could not fall into disuse and be subsequently reinstated. The difficulty of proving this over seven hundred years led to a change in the nineteenth century. It was established that any right which had been continually exercised for at least twenty years (or thirty years where the right had been exercised against a claim by the Crown) should be regarded as having existed since "time immemorial".