Well I finally caved and realised that:
a) my screw splitter is not the safest thing around
b) running a 80hp tractor just to power said splitter wasn't really smart
So, tractor and splitter are on eBay (and will sell for more than I paid for them) and I've gone and bought a petrol splitter.
It's great! Horizontal for small stuff and vertical for big stuff, runs on a sniff of fuel in comparison to the tractor and I can tow it around. 20 ton pressure for those that care.
Bring on those 3' rings!