Thank you Bruno, much appreciated.

Looks like you'll have a the first test run in few days time judging by the current rate of knots? 

Hope so, I'm intrigued to see if it'll run!

I was planning on using the lathe to drill the offset holes for the crank-pins but trying to use the spider, some shims to protect the edges, adjust the jaws and leave enough space to fit in the DTI was too fiddly (must be having a King Charles sausage fingers day). So I resorted to the wobbly old pillar drill, putting the workpiece(s) down at the bottom of the vice which is the only place it's likely to be anywhere near perpendicular to the drill, and kept everything clenched. In the end they came out fine; I put some long screws in to check for squareness by eye and they look pretty good. Next to make the pins and the connecting rods. Hopefully the screws I'm waiting on will arrive tomorrow and I can get rid of all these temporary fasteners.

Looks like you'll have a the first test run in few days time judging by the current rate of knots? 

Oooh, looking good!

there's a similar engine here based on a supercharged twingle

https://en.wikipedia.org/wiki/Zoller_supercharger

I really like the twingle, they're so simple, i'd love to own one some day.
If i was to modernise the design, then a screw type supercharger would be the way to go
and i would arrange the cylinders 45 degrees across from each other, so as to allow the use of
two con rods on a longer crank pin. the master/slave type are unreliable.