When I found this, my eyes went WOW! I can't believe it! This is taken from a concert at the Montreux Jazz Festival in 1968. The sextet has the following line-up:
Mike Osborne - alto
John Surman - baritone
Malcolm Griffiths - trombone
Mike Westbrook - piano
Harry Miller - bass
Alan Jackson - drums
This concert features the line-up at about the time of the first Concert Band albums on Deram from 1967 onwards to about 1970. I can't identify the tunes here - two slabs at about 20 minutes each - but I'll have a listen to the afore-mentioned albums and see if I can get any further. In the middle of the first piece, Surman slips into "The Girl from Ipanema" of all things (which is on Release) and there are some thematic passages that do sound like marching songs. To be verified if possible.
Make this an essential listening if you're into golden age British Jazz. And I mean essential!