The mystery is solved.
I ventured to the loft and dug out my old box of memorabilia and there was the offending ticket stub for..............An Evening With Cliff Richard on 17th September 1973 at The Royal Festival Hall.
Many thanks for taking time out to search your records it is much appreciated.
It does, however, leave me with a dilemma as to why for so long I have this enduring memory. On that basis I have booked myself into rehab as a penance...... I may be some time!
It means I must have seen the concert at the de Montfort Hall in Leicester as we were living in the East Midlands at the time, so at least it fills in a blank.
Many thanks and apologies again.
Graham