I finally cut the thread, it was done with a HSS tool at my lowest speed (65 rpm?) using tapping lubricant. The swarf came off in continuous curls, but the surface was rough and torn. I cleaned it up a bit with, as suggested, a piece of hardwood and then followed with the folded edge of some emery cloth. The nut fits nicely
I bored through the centre up to 16.3 mm with out too many problems. The 40mm overhang meant I got a bit of chatter, but it's not an important bore.
I have now set up the taper using my MT2-ER25 chuck and will try and bore that tomorrow.