Step 5: Welding
The nuts just need to be tacked onto the tubes. Try and keep them centred, otherwise the thread of the locking knobs will scrape against the side of the hole. There is a cure if this does happen, use a 5/16" tap which will cut a thread into the side of the hole.
The pictures show the welded jigs.
Thanks to my Uncle Peter who welded the bits for me. He did a great job.