This summer I have done a lot of mileage and seen a lot of Magnettes. On my travels I have found two examples of metal-fatigued wheels, both of which might have been subject to catastrophic failure at high speed. What really caught my attention was that one of them was on my car!
Magnette and MGA wheels are known to be prone to cracking if used "enthusiastically". Some may even fail in normal use simply because of the age of the metal. Just think about the lateral stresses that they undergo when cornering.
Check your wheels now. Take off the embellishers and look for hairline cracks in the area between the wheelnuts. If a linear stain won't wipe off, the chances are it's a crack with dirt in it. If you find cracking, the wheel should be condemned immediately.