Cottage pie seems to have gone wrong

Thanks for your response. W'll take on boars your useful advice. As far as tossing it is concerned, that was done fairly quickly as it smelt vile , my wife tasted a tiny bit and that was vile too.
Thanks again.
Regards,
Andy

What you have there is a good example of a bacterial contamination of some sort. Without a lab, it will be impossible to identify the precise culprit- and they are fairly numerous. It could be due to the mince, the gravy, or a sneaky critter on a spoon, the container- who can say? If it didn't cool fast enough, or stay cold enough- they had a great shot at doing what bacteria do, and multiplied. They ate a great meal, and then did what a lot of us do after the great Turkey day feast. They burped. (polite language here- you could use other terms) It's not fermentation as in producing alcohol or pickles, exactly- because they are not the human friendly sort. Gas is a byproduct of many living organisms, which can be trapped in the surroundings if it is thick enough. At any rate, your cottage pie filling is not likely safe for consumption. I'm sorry, but your best option is to toss it. In the future, you just want to be sure of your storage temperatures, and get things cold as quickly as possible.