I think it may be possible to push the mast up through the bottom rear of the front wing and then hopefully push it out through the wing into position.
I hope that fixing it does not rely on gripping the underside of the aerial whilst tightening the top nut. I will give it a go when the replacement arrives.
Just finishing this off as the new aerial is in. There is a gap at the top and bottom of the wing next to the A pillar. I used a coat hanger wire to pull the cable up first from the bottom and out into the engine bay. Then the same wire attached to the top of the mast to bring the aerial up to the mounting hole in the wing. The aerial cable then went through the bulkhead with the main engine loom. It did it myself, but it would have been a lot easier with a helper.