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It occurred to me that good old fashioned baking soda might do the trick, then I 'googled' the problem. Blimey you would not believe how many people have had the same problem! However the concensus of opinion appears to be 'keep at it' that one 'soak and scrub' session is not enough. One suggestion was to leave it lying on its side in the garden for a couple of nights and slugs and snails will do the job (have no idea how much reliance to put on that one).
Baking soda works pretty well for cleaning ovens. Pour a thin layer of baking soda in the bottom of the pot. Spray some water on it and cover it so it doesn't dry out and let it sit overnight or for several hours. It may not get it all, but it usually gets enough of it that you can scrub the rest out. Or try it a second time. Good luck!
All we use around the house is Baking Soda & Vinegar. We have used bleach the odd time but mainly have stayed away from most chemicals for 20 years & haven't died yet!