I know Harrow quite well, but am too young to remember your time period. To try and help if you don't get any more leads, there is a local paper - the Harrow Observer you could maybe email them a copy of the photo and ask them to publish it for more details or another line to possibly follow is - on the picture there is Harrow 6650 which I assume would be a telephone number. I have tried Googling it, but to no avail, however there is a BT archives which you can visit and maybe find out that way. Hope this is of some help.