Sorry, have been sorting dinner/bedtime so there's a lot to reply to now! I'll try and answer all the points though!
Petrol - DH and I don't drive. DB and his GF are taking their cars and my Mum is taking hers. She has actually said she doesn't want to be given any petrol money so we're splitting the cost of two cars between us really. DB's GF lives in Winchester so will actually have less of a journey than the rest of us (from Surrey) but we're not deducting petrol money for that. And, obviously, none of us are actually going in her car, so it's a bit of a win win for her!
Rooms - yes, the boys will be sleeping in with us, hence why we have divided the rental price between us this way. She was also the one who originally worked out how much we all needed to pay so if she felt it was unfair she could have worked it out differently then.. we all just pretty much went by what she told us.
How much they normally spend on food - They eat pretty well. Sainsbury's finest range, or whatever it's called.. they drink expensive beer and wine, etc. She eats a lot, and likes to eat decent food, so if they cook it will be posh stuff not ready meals and stuff as she's a total food snob. She won't eat cheap cheese/cheap bread/drink cheap tea/etc. so she will probably actually cost more than everyone else!
Our DSes' food - I said in my OP that DH and I said we would be contributing extra for the boys. DB and his GF know this, it's already been discussed.. they said they thought an extra £20 or so would be alright. This is mainly because if we have a takeaway the boys won't be having it, unless it's fish and chips, so would just have a quick pasta bake or something earlier in the evening. Imperial We're not expecting anyone to 'subsidise' our children, we asked them how much they thought we should add for the boys and they said £20 or so. DS2 doesn't eat much at all anyway, may as well still be EBF some days, so he might not cost much at all! And obviously we will pay for nappies, etc, from our own money.
I know £50 sounds like a lot but when we first agreed on it we decided it would cover fish and chips the first night, which is a bit of a tradition, plus a food shop (cereal, milk, meals, bread, loo roll, etc, etc) and a takeaway at some point during the week. I didn't force her to agree or anything, she and DB both said they'd be happy to do it that way. She is the sort of person who likes to get as much as she can while giving as little though.. she invited me to go shopping with her once then told me I had to give her money for petrol!
I guess I am being a bit U because I could fairly easily just say to her that we'll sort out our own food and she and DB can please themselves, but I just sort of feel like she's getting a fairly good deal with the petrol thing anyway so she should really stick to the original agreement.