Hi guys, thanks for your interest and a couple of possibilities
a/ raceinstructor on karting1.co.uk suggested it could have started life as a Zip Californian circa 1971 before a bodger got hold of it
b/ I found a picture of a Barlotti Class 4’ 250cc gearbox kart chassis that looks similar
either way I'm going to chop some bits off it - put the rear axle over the chassis, chop off the lower arms to the king pin, in an attempt to put it back to something like a go kart