Are you getting Universal Credit (UC)? If not, then apply for Universal Credit.
How old are the teenagers?
Claim Child Benefit - if ex was on a high income then you may not have been claiming.
You can get the child element of UC for 2 of the teens if under 19 and still in full-time education.
Do you rent or own? If rental, you may get the housing element of UC.
Depending on income, you may get a reduction in council tax.
Cheaper broadband is an option if claiming benefits. They are called social tariffs.
Water rates are cheaper if on a low income, and the income threshold depends on the household size. Eg Welsh Water have HelpU rate for low income. There is a capped rate if in a meter and more than 3 kids/someone has a disability that means more water us used.
In Wales you can claim Education Maintenance Allowance for low income household and over 16 in education.
Depending on age of kids, they can get Saturday Jobs.
Selling old books/games/dvd on somewhere like WeBuyBooks.
Make sure you're getting the Maintenance from the ex.
Saying no to teenagers!
Pocket money dependant on pulling their weight around the house.
Buy bulk. Packed lunches. I put the crisps/breakfast bar, fruit etc ready on a Sunday afternoon. They make a sanswich in the morning.
Energy saving settings on dishwasher/washing machine.
Using a budget planner, and following jt. Making sure you save for the big stuff like car maintenance/insurance/Xmas through the year.
Saving for insurance so you save the cost of paying monthly.
Cancel all unnecessary direct debits.
Netflix Or whatever- don't have them all. Take it in turns and have a few months then change.