the chainstay is fine - it's the drop out which has snapped. Unless you had access to a frame jig, 'putting a new chainstay in' is not really an option, not to mention unnecessary, not to mention that finding a replacement chainstay or fabricating a new one would put the cost well above £50.
If this was a vintage Colnago, Rossin, De Rosa, Daccordi, or maybe a Hetchins or something else collectable, it might be different. But it's not - it's a mass-produced Taiwanese MTB frame from the 90s. Seriously, take it to the tip and move on...