It is apparently a hand-made frame by Bell Street Bikes, London.
It all started back in '87 when I worked in Bell Street Bikes NW1 after school building bikes like the BSB400 touring bike and the BSB Blazer ATB which was a 531 frame with TA chainset, Dia Compe brakes and Suntour index gears, this sold for £400.
I also found a comment on a Guardian article from 2011:
Geoff [the builder of the bike] passed away last year. Complications of leukaemia finally took him, even though he had battled it twice. A great guy, a funny man and someone who cared for other people. I used to work with him at Bell Street Bikes and it was one of the happiest periods of my life.
I'm very pleased to have to have learned more about it. I inherited it from a friend of a friend, who also died from cancer.
It has always been "Gary's bike" to me, even though I never met Gary. It's a lovely bike that I have spent many happy hours riding. Here it is half-way up a Spanish mountain (it's the one with yellow panniers):
My plan is to have it resprayed and rebuilt.