[Dave80GTSi]

Update: Sorted it, but Geez Louise, what a pain …

Tried a number of ideas, and here's what worked best for me.

You need about 4 small clamps and a heat gun. Starting at the crest's bottom 'point', clamp it correctly to the gasket. Going around the perimeter, use the heat gun to warm the next little bit of the gasket, and clamp it into place as you go along. Once you make it back to your starting point, you use the heat gun to warm the entire gasket, and then leave it to cool to room temperature. By the time that you've done 3 or 4 heat / cool cycles, you have a short window of time to remove the clamps and to quickly install it to the car, with the clamping force of the crest's nuts to hold it all into place.

All in all, far too much fiddly time and effort than one should expect to install this common part.

After all of this time, I can hardly believe that one is expected to go thru this sort of exercise just to fit a simple gasket!