As Chris pointed out, I made a bit of an error with the arms. Today I set too to correct it. But how to drill holes in the other ends accurately? I don't possess vee blocks. Can't do them in the lathe. So I had to think of some other way.
I finally did them as below in the photo. Wasn't exactly easy keeping things square, but taking my time things came out OK. I used one of the extension sleeves as a guide for a drill to spot the centre. Then I drilled the spindle using three different sized drills, finishing with the tapping size.
Next the holes were threaded 10mm and the stub axle on the arms cut off to 3/4" long. See photo below.
The next job was to shorten the trailer stub axles to 1/2" long. Cut them down to 5/8", then into the lathe to be faced down, drilled and bored out 3/4" I.D. x 1" deep.
Below all ready to weld up.