Right...first of all, thanks for the replies!
I've been connecting the battery to the chassis and the +ve feed that goes to the starter/alternator. I have tried with it connected to the gearbox and starter motor directly with no improvement.
It would seem that the engine is just a little bit tight after the rebuild. With the sparkplugs out, the starter has enough oomph to turn it over, but it's labouring. Tried removing the aux belt at well, but it didn't make much difference.
I'm quite surprised, because it didn't feel excessively tight as I was building it back up, although the crank thrust bearings were quite a tight fit.
I have 2 plans... 1) Jump 2 (or more) batteries together in parallel and see if it'll go, or 2) tow it along in gear for a while to get a bit of oil circulating, then try bumping it.
Anyone got a better idea???
I think that if I could get it started, and let it run it for a while, it would be fine. I really don't want to have take the engine apart again (or even the sump off, which is a pain in the ar5e)!!!