As far as the mix is concerned it depends on the oil you use. The original figures for the old strokers are based on standard engine oil and shouldn't be adhered to for modern synthetic 2-stroke oils. I run a Villiers 1H engine on 40-1 with a synthetic 2-stoke oil, but it can vary slightly depending on what grade/make of oil; you'd do well to find someone who regularly uses a Villiers/Bantam and find out what oil and mixture they use. At any rate modern oil is much, much better than the original 'crudes' they were running on so it won't do much harm to run it rich and gradually use less oil until you find a suitable ratio.
The overheating problem is unlikely to be caused by the petroil unless you're running it very hard and lean, which it seems you haven't. Has the barrel been lacquered/powdercoated/enamelled? Sometimes -depending on the coating material - this can insulate the barrel itself and prevent the heat dissipating properly; not normally a problem on four-strokes but I've heard it can happen on strokers.
Have you checked the exhaust pipe? If that's full of carbon and choking up there will be a lot of exhaust gas (ergo, heat) being contained in the cylinder.
Aside from this all I could think of, as Harry pointed out, is that someone may have fitted a piston/rings too much oversize for the bore. It's not highly likely, but stranger things have certainly been known to happen.
Alternatively you might be leaving the choke on too long, but I can't imagine that being so detrimental so rapidly.
Hope you get to the bottom of it,
Blue
