In the end it seemed to go together ok. There is a tiny bit of play on the spindle, but not as much as I thought. Part of the problem was the crank arm cotter pins weren't in properly (they needed filing down a bit) when I did a dry fit before so the crank arms wobbled. I would still like to take it apart and run a tap through the thread though.
The other side was much easier as it had two flat edges on opposite sides, so a mole wrench gripped it and tightened it up as far as it could go.
I had seen that Park tool on-line, but I wanted to find one locally to measure it up first.
Thanks for all the advice. The bike is pretty much finished - just waiting on the brake blocks and some new decals to arrive.