HB, enough is enough. You're going to end up having not so nice memories of your mum, because she's wearing you down.
How about her chosing stuff from somewhere like M&S. Maybe that would be more to her taste. Microwave, or ovenable stuff she can heat up. Mind you that would mean a trip to your nearest branch, not overly sure which is your nearest branch.
Won't be easy, but you need to be firm. Tell her you will sort out Wiltshire Foods or wherever to get her meals from.
I agree get your two to visit her on Sunday, so you can rest. You're a mum too, so it's your Mother's Day too.