I regret not taking photographs of this as I was so impressed with the results. Being a scummy gingerbread mummy I have browned meat in my pressure cooker a few times instead of using a separate pan. The result - a stained and marked, horrid looking pressure cooker that wouldn't scrub clean.
The ever useful YouTube and Google gave me the answer - citric acid - the stuff I happened to have left over in the cupboard from cleaning the sterilizer when it was still in use.
15 minutes later I had an immaculate pressure cooker with very little effort.
Take one sachet (10g) of citric acid, add water up to worst marks - 2 to 3 cups fulls and boil for 10 minutes.
Empty, and then clean with a scourer and washing up liquid. Job done.