You need to get a handle on your spending to understand where it goes.
A clean sheet of paper and write down all of the outgoings then go through your statement to see what you spend on your card. This gives you a vague idea ( lots of cash spending you won't be able to account for)
After pay day i would suggest you go cash only for a month and jot it all down in a note book. ( then you know exactly)
It's a very simple way to work out what you fritter money on. Sky, phones, coffee, school and activities all add up.
Once you know how much you have to spend and roughly what you spend now, you can budget properly. If you only have £100 a week for food, take that in cash to the supermarket. It will focus your spending. Get to aldi, food is one of the biggest expenses after rent.
Alvin halls book is excellent, see if you can get it in the library.
Working out spending as a % of your earnings is often shocking. £600 a year on sky is a big chunk of your money.
I don't think it's a lot of money for a family with three children in SE.