A few minutes later, or maybe more, I came back in, and my nose said that's it...I was resolute, I would not be upset. After all I have had this pan over twenty years, and maybe another one would be best. There is a slight dink in the side when someone, not me must have dropped it!
The next morning lid was completely clean, but the pan was still solid. I trailed the internet, and decided to use the vinegar and baking soda routine, together with plenty of elbow grease. Actually I used an old spoon to scrape the surface. I did this in batches, as my elbow grease was running out and finally the pan/casserole is useable again.
I've just looked up the price in case I still want to buy another of these stellar casseroles, but no I won't be, and I now think the time I spent scrubbing really worth while.
Prologue: I found a shop just by accident on a day out to Stow...and they had a Stellar caserole, of a large size, on offer, at a price I could not refuse! I still have my smaller caserole, and I have since had the tip off a friend that rhubarb boiled up in the pan works too.