After a lot of excellent advice about processing Mercury from another UK based imager, I was able to pull a fairly decent image using AstroSurface with the TIFF straight out of AS3!, without going through Registax. I also culled the stack to just 20 frames from the 28,000 captured in the SER video. I couldn't focus on Mercury at all, so used Venus as a focus target and captured a single SERs for that.

I have done I've present both planets together at the same image scale, though of course they are nowhere near that close to each other in the sky. Mercury is much further away on the other side of the Sun in its orbit, hence it shows a gibbous phase, whereas Venus is now racing towards us, coming between the Sun and Earth, hence it's slender crescent phase and much bigger apparent diameter.