I can't offer an opinion, as I'd like to express my interest for if/when it's up for sale... however talking in abstract I would first check current selling prices for one in good condition, and then consider the knock-down price you'd expect to pay for it on a stall at Kempton.
Edit:
For interest, a 'good working order' had an asking price of £125, but no idea what price it sold at...
https://www.gunstar.co.uk/original-m...r-Guns/1636325