I got some plastic resin in the post today and did some experimental casting using my existing moulds.
These were the ones that worked out best, not much flash and the pike on the Sumerian cast perfectly. They have turned out a little tacky, I think this is due to either not measuring the quantities of both fluids well enough, not stirring thoroughly enough, or the temperature being too low. Or maybe all three. I've ordered disposable measuring cups and proper stirrers and I'll keep an eye on the temperature in future.
I had problems casting mounted figures, the tail didn't fill, probably because it was positioned such that it caused an air pocket to form. The same issue occurred to a lesser extent with the bases of these units. This wasn't a problem when casting in lead, presumably because the weight of the lead forced them to fill. Some figures didn't fill completely, perhaps because the mould fills through too small a hole at the end of a funnel designed for lead casting.
The one big downside with resin is that a failed casting is a waste of materials. With lead I would just melt it down and use it again, that's unfortunately not an option with resin.
So, lessons learned. For resin casting I will make special moulds. There will be run offs to stop air pockets from forming and the figure will fill directly through the base. And I'll make sure I measure and mix properly with the room at a reasonable temperature.
I won't mix resin and lead within a range, I don't like the feel of using both heavy and light figures during a game anyway, it has to be one or the other. So resin will be for new ranges I make for now, and I have a particular idea in mind for a child friendly range, possibly a Christmas present for my Grandson.