Comtesse - please don't worry about it! I am grateful for your advice. 
They are not aggressive or horrible deliberately - a lot of the stories on here are far, far worse. They are just incredibly insensitive, very patronising/bossy, and they don't 'do' emotion.
OK, so this is just me venting now... but booooooy does venting feel good right now. Bear with me.
Language, to them, is a set of noises that are uttered in order to prevent there being any silence - very little is ever actually communicated. Don't get me wrong, I'm not one of those people who looks down on small talk and thinks that everyone should be talking about Tolstoy or not speaking at all. It's more that they actually don't converse. They fire information/commands AT you, and never expect you to have any view on anything at all in return. Sometimes DH and I try to exchange a few words of a practical variety (did you turn the gas cooker off? No, I thought you did that? Oh shit, do you think we should go back and check?), and MIL will actually shout over both of us, and then make a kind of inane joke that she has clearly thought up on the spot for the sole purpose of interrupting (my favourite has to be the time where she repeatedly said my husband's name six times in a rapid crescendo to volume:foghorn, and then pointed out a traffic light). It is odd.
When we go to their house, we do exactly what they want, when they want to do it. When they come to you house, we do exactly what they want, when they want to do it. The idea that others might have their own preferences, thoughts, ideas or inner lives does not occur to them. They were massive bullies to DH when he was growing up and it has taken him a lot of therapy to get over it!
But I need to get over it and get back out there because I have another day and a half to spend with them, and DH loves them in spite of it all. So am now going to sit and think positive thoughts and try to remember it is MIL's birthday and she deserves to have a nice time. I have made her a massive chocolate and coffee cake, so I will go and light some candles, hitch on my best smile and crack open some champagne to celebrate her special day.