Thanks all. I shall try adding a 'pin' and balancing them. Might have to wait a few days before letting the wind have a go else they might just fly ...
Well I added a pin and checked the balance, not surprising one side was heavier than the other.
Added a couple of bits of scrap to the other side, and it was lovely and even.
Linished the whole thing to get rid of the many sharp bits … and that side is now slightly heavier Still I can adjust further if I feel the need. Beat out a small 'dish' out of scrap and welded it to the top of a bit of 6mm rod.
Bit blustery today (but so far better than forecast), and the rod might be a bit too flexible, but proves it should work.