I had got the other one off turnwise with some effort so presumed the replacement would be the same. It did seem quite tough so I probably did trash the thread in the process.
Anyhow, I baked a cake for the final winter track meet tomorrow. My estimates (9x12" pan being 3x the batch needed for a 7.5" round tin) whilst mathematically correct appeared to have ignored depth. It took some time to clean up the overspill, and the cake is a bit uneven, having been cooked in the top oven (no fan) whilst waiting for the insulator pad that I didn't know I needed till removing the old fan and having it crumble to pieces on me.
£75 in parts and delivery to repair the oven.