Sorry Al didn't see that post either
I am def not as good as keeping up with the thread as I used to be but am much busier these days!
How many people/kids are you budgeting for?
The way I do it is to go through all the monthly direct debits and tot them up, look at any I think could be reduced by switching companies or using less water/electric etc.
Then take that off my total monthly income and work out whats left.
I roughly budget £40 a week for food and £20 a week for petrol, household items probably come in at another £5-10 a week depending on what we need. I shop in Lidls a few times a month and always look for offers and am not loyal to any brand other than Heinz beans
I then spend on clothes, toiletries, going out and anything else depending on what is left. I am naturally now quite a cautious spender I think and won't buy something if I think its too expensive!
Thats for me and 10yo dd (and 2 cats!)
Em your mil sounds like such a nightmare, I think your plan to enjoy the holiday regardless is fab though.
Life do not beat yourself up at all, dd has often made 'potions' and its pure luck she hasn't tried any! Georges Marvellous Medicine has a lot to answer for 
Right I need to get off the laptop and start sorting myself out, car needs emptying of work crap and repacking with stuff to take with us. Will be off the thread again until the end of the week, we get back Thursday.