I failed to mention the other not available part....the fine thread flange nut, the ring which sounds to be, in your case, stripped. This is as you observe possibly a stopper, at least for a while. By the sounds of the base condition I was about to suggest buying a new base, which would fit the old machines, but now there is a bigger problem...that ring is only available as a used part.
We have brainstormed over this problem many times and have seen a home made flange nut in the fine thread but it was a labor of love part that would be far too expensive to manufacture in small numbers. There are some extreme approaches to the problem that come to mind but once again these are for the hobbyist that has the equipment and is doing the repairs for the challenge and the economics of the affair is not an issue.
This will take a bit of thought.

Thanks for the help and information so far, I really appreciate it.

I think I will persevere with removing the element. Only then will I be able to decide what I am going to do next. It seems such a shame.

I am wondering if I could use the flange nut as is (without thread) as a spacer, and use the element to hold the boiler to a new base (with extra spacers as required), maybe an extra thick compressible seal that will allow the element to seal at a range of tightnesses. Without having the parts seperated, I can't really see if or how it could work.

As you say, a bit of thought

Wait a second, wait a second, wait a second! I am about to repair a 220v 1979 unit. I live in a 110v country. I am going to probably buy a new 110v element. To make this work I will need to buy a new flange with the coarse threads, rendering the one I have here "extra". Would this be correct Doug?

1979 sounds a lot like a fine thread boiler base needing the fine thread nut although I am not sure what year they changed the thread. Acccording to Franceso's site you would have the pot metal fine thread, then came brass fine thread then bakelit and the modern bras coarse thread http://www.francescoceccarelli.eu/La_Pa...20ring.htm and he would know. I know your workup of the 79 is on another thread and you were going to try for a bolt on changeover, good idea, but still that available coarse thread nut vs unavailable fine is an issue.

My nut does look like the second in that link. It says steel, so I am hoping that the steel has corroded and the boiler thread is still intact. Does that sound likely? If so, there is still hope for me!

I will follow what you are doing with interest. If you do manage to find the fine thread brass nut, then I would be very interested in your old steel one. Of course, if you manage to find someone that will make you a new nut with the correct pitch for a reasonable price, let me know!