Hi Martin,

> Using the Shell tool to thicken it up a bit, gives a rough result and
> I wonder if this can be tweaked.
>
> The other cone was made from a revolved line and the surface thickened in the
> same way as before, but this time, has turned out as expected.

I've got this updated for the next v3 beta - in the next beta when using Extrude to point where the base curve is a circle or arc shape, it will make an analytic cone surface out of the result and that will help this case to offset properly.

Thanks for reporting the problem,

- Michael