Dh offered to pick them up and drop them off so that they didn't have to drive (roads are still dreadful around here). He didn't mind not drinking as he had a stinking cold and was taking Sudafed/paracetamol.
I advised of "our" routine for the day (when the invitation was first extended and accepted.
We have a lazy cooked breakfast in bed at about 7am; whilst we open all the stockings and then just a large buffet at 1pm. (Game pie, ham, pate, cheeses, hot French bread etc)
We then have dinner at about 8pm. When I told her she said "That late" I confirmed that that's what/how we do it. (over a month ago; she didn't decline).
Dh said he'd pick them up at 12.45 and buffet would be ready when they arrived. They phoned at 12.25 to say they were in the car and on their way. 
. DH had spent 1hr digging out car/running it /getting it warm/clearing driveway so it was safe for her to walk etc.
They turned up, served buffet. Mother asked what time was dinner being served as she stuffed her face. I repeated "about 8pm".
"Oh, that's too late; we're leaving before it's too dark, we don't like this snow and driving in the dark; that's just a ridiculous time of night to be eating" 
.
Bit tongue, went to my home office next door to look at turkey in the fridge in there. Still not completely defrosted; and all the pipes had burst 
.
Just said; "Well sorry, turkey wont defrost before the scheduled time, and I'm not risking cooking it, until it is. You didn't have to drive and you knew that and the way that the day was planned out. You must act as you see fit; alternatively the guest bedroom is all freshly made up and you're welcome to stay and drive in the morning."
They left at 4pm.
I plated two Christmas dinner's (and pud) first, before I dished ours up, and DH drove them to their house, with full microwave instructions, and individual tupperwares of cranberry, cream, brandy butter, brandy minature and a box of matches
.
I felt sad for them. My home was happy, despite them, and the love/affection that my DD feels for us both (despite rapidly turning into Kevin) is obviously difficult for them and a bigger poke in the eye/heart than any of my words can say.
I was a charming hostess and they got a fucking Christmas dinner. It was all very lovely , although not entirely as planned..
DH said I was very brave; and very good for not slapping the "Bloody obnoxious, ungrateful Woman"