Barry, thank you very much for your reply.

I have a couple of questions if I may:

1. You mention 'move the seam to the inside by rotating' - could I ask, the inside of what and how do I rotate them in that plane?

2. To generate your base surface get the calc lengths of the top and bottom ovals and sweep curve length. Is this the length you mean - see screenshot.

Many thanks in advance.
Gunter