Sorry I didn't update sooner but my thoughts have been a bit disordered.
My dad wanted to talk to DH because my mother had been going on about him abusing me, and naturally my father was concerned. DH just asked my mother which behaviour of his she'd characterise as abusive. As I said, she knows next to nothing - so she squirmed a bit and just said he has a lot of control in the house as the provider, and that clearly he's been abusing it as he's stopped me from talking to her.
DH replied that I could make his life just as difficult by pulling out of everything I do around the house and that neither one of us has "power" over the other. But, more importantly, he said that he hasn't stopped me talking to my mum at all, and that everything that's happened has been my decision.
My father was clearly pissed at this. What the hell I was playing at, she raised me, she loved me etc.
I said I had children to raise and love and had to put them first, and if my mother is incapable of conducting herself safely around her grandchildren then I have to protect them. My dad asked what I was talking about; he didn't know about the phone call!
She pulled a lovely 'shock' face and then essentially insisted my DD was making it up as an elaborate ruse to get out of the holiday. My dad said that we'd have to discipline DD for dishonesty, DH said we would no such thing as she wasn't lying!
My mother said she was. I pointed out some key incidents from my childhood in which I'd been accused of doing similar when my mother had behaved inappropriately - and punished for it. Besides, I'd picked DD up in total shreds so I find it very hard to believe that nothing had happened to her. My mum had also definitely called her about something, it says so on DD's phone!
She then backtracked and said she had called her, but just to check all was okay and as she was very oversensitive she had gotten upset, not because my mother was purposely trying to upset her.
Obviously at that point it became clear she was lying as she had previously denied the existence of a phone call, as DH pointed out. She tried to offer some more explanations like DD had got a friend to do it for her etc., and changed the contact name 
DH said but you admitted you called her.
She then insisted I was being confrontational before it even happened, my dad said I'd upset my mother very much and I said she'd upset me! And showed him the texts. She said she hadn't sent them and showed us her phone (texts deleted of course) and it was the contact name thing again, so I read out the contact number.
My dad looked quite shocked, my mother spat at DH! She said he'd clearly got me very well trained, and then went into this rant about blaming herself for me being so stupid and weak etc. Other people in the café could hear her and it was very nasty so my dad told her to be quiet and they ended up having their own mini argument.
At last my dad said (to me) that my mum had been inappropriate but it was just out of worry and maybe her MH isn't what it used to be. I said it was exactly as it used to be and now that I had kids of my own LC was the only option, and my parents wouldn't be coming on the holiday.
My mum kicked off to the level she was asked to leave, so we left too. My dad said he would call tonight to talk to me alone.
I'm feeling very confused and disordered about it all still, it keeps replaying in my head. My mum looked positively unhinged and my dad found it do easy to believe all her lies. I get that you might believe your wife over a teenager, but how could he believe her crazy explanations over the truth?
I've got to try to get my head in shape before tonight, DH said he's happy to be there with me on FaceTime or something but my dad did say alone. I'm not sure if he just meant without my mother. This is just all so ridiculous. It's a bloody holiday and she's somehow turned it into this.