@Happbee
Can you please tell us who was at the dinner party as quite a few people have asked and it's never been answered.
Also, you say DH was invited as it was happening. What do you mean by this? Did MIL call him and ask him to come round or did she invite him to stay when he was already there dropping off the pudding?
Doing a family member a favour like this is acceptable and I wouldn't ask my mum etc for the money for the ingredients but she would offer and thank me and that makes the difference.
It's different if you don't get on with MIL and especially since she won't set foot in your home and she's no time for you on the phone, it's kind of rude to be expecting favours from you when she's no time for you. Did she call and thank you for the pudding at least? I suspect not but you haven't told us.
I'm assuming your husband has noticed yous don't get on, does he ask his mum why she doesn't come into your home? Do you ask her at the time why she isn't coming in?
I don't think you should be offended you weren't invited, it's a normal thing to ask for a favour but if DH was invited anyway then I'd think differently about it. I do think she had the cheek to ask for this favour when she's no time for you and I wouldn't be doing it again. If you have no time for me you have no time for my pudding.
As for DH, let him be annoyed when you refuse to do his mum a favour. You shouldn't need excuses but next time a favour is asked you can say no your last favour was shown no appreciation. If your still going to be a doormat say sure, tell your mum to give me a call direct with her requirements.
Oh...and stand up for yourself!!