G-AWBL Obtained 1/5/68 for Autair, to CourtLine 5/12/69. Disposed 20/1/71
G-AXML obtained 30/4/70 Leased to LANICA 22/12/71
So the photo was taken between 30/4/70 and 20/1/71. Seeing that there are 4 1-11's in the maintenance area I would guess the photo was taken before or after the main summer holiday flying programme for 1970.
If I had to pin it down further, looking at the shadows the sun is quite high so I would say first week in May 1970