Not sure its a Matchless but it could well be Coventry Eagle, who built bikes using quite a lot of last years Matchless parts; OEC did similar, the tank does not look Matchless, but very much Coventry Eagle so thats what I'd plump for.
Would also throw OK Supreme into that hat as well.
Yes that distinctive long tank and the rake of the forks match, plus I'd say that with that gearbox, not one of Burmans best, and coilignition its a 250cc.
Matchless supplied engines, and often frame parts to several other marques, mostly obselete or nearing the end of the range stuff.
They'd supply timing cases at odd angles, and with appropriate motifs cast into them, also the rocker cover plate would have the logo or just fins cast in.