Where on 1st July 1948 land was normally used for one purpose and was also used on occasions, whether at regular intervals or not, for another purpose, planning permission is not required in respect of the use of the land for that other purpose on similar occasions on or after 6th December 1968 if the land has been used for that other purpose on at least one similar occasion since 1st July 1948 and before the beginning of 1968.