My first ever concert was in 1978 (I was 18), it was The Jacksons at The Rainbow theatre which, if my memory serves me, was in Finsbury Park. Small venue, great concert!
This should expose the varied demographic that is PH!
I think mine was probably someone like David Essex or B.A.Robertson, c.1977 or 1978 My eldest brother's best mate's related to Mel Bush, their promoter, so we used to get freebies.