It's in! Still took a couple of tries and you're right Nodge, the outer splines weren't meshing. It didn't help that I hadn't put the gearbox into gear when I was turning the engine. Once I did that I could see the IRD give a little kick when the splines meshed. It pulled in easily on the bolts then. Just everything else to connect back up now and refill the fluids. Thanks to all who replied.
Now, any tips on how to bleed the clutch? Doesn't look like you can connect an Eazibleed to the reservoir. Is it just the old fashioned pumping the clutch pedal method?