Tuxford,
what I mean about tightening is not so much cranking down until something brekas, rather tightening every day or often. The aluminum is so soft even a small turn of the wrench will squash the gasket. Little by little the gasket will deform. I don't know if this is happening with yours or not. As thick as the gasket is and as much meat as there is around the center hole and the bolt holes, it doesn't make sense why the gasket would start to move. I do know I will be getting a quote for the next batch in copper.
Anyways, I do need your address as you are not in my UPS database any longer.
Please email me directly and I'll get another gasket out.