I'm going to come at this from a differernt angle, because I have no interferring mother to rant about and mother in law lives far away, well out of fighting range.
Just to say, and call me a sentimental fool, no matter how meddling the mother, at least it's attention. When my son brings back yet another picture from school, or the toddler increases his vocabulary to two whole words, they can only rely on me (and my husband) to praise them. OK, friends are interested in a PC/polite kind of way, if they happen to be around, but they are hardly waiting with baited breath to hear how the potty training is going, and they are not going to swell up with family pride if my older son gets 5 out of 5 in his spelling test.
Friends with their busy lives have not got the same urge to give and to hear a running commentary on my familys' goings on. Sometimes ( not always!!) I would give a lot to have an aunt or an uncle, a mum or a dad pop round to get their regular fix of our family progress.
As for the power struggle, MaryLou, I think you've been given lots of good advice. All I can add, from a posiiton of some innocence, is this:
Could you open up a discussion with your mother by saying you really think it's important that all the adults in their lives present a united front to your children. Hopefully your mother won't disagree with this. Then outline your very general view on important areas like eating, sleeping, smacking etc. Hopefully you can reach some very general agreement with your mother on this. Then write it down and show your mother.
And then if she ever oversteps the mark in a big way, find a method, as Kia suggested, of tactfully reminding her. As for the little things, let them go, especially if she is doing them if or when she has your children, and you are not around - all free baby sitting is to be encouraged!!