Phone Gingerbread, they were amazing with me when I was leaving and helped me to understand what I was entitled to apply for.
It will depend on your salary but if, for example, you're earning minimum wage, you will likely qualify for certain help - possibly 70% of childcare costs, Working Tax Credit and Child Tax Credit through the Tax Credits office. Child benefit if you're not already receiving it. Housing benefit and Council Tax Support if you're renting.
At the moment, the help available to a single, working parent is reasonable, though I've read various things about possible future penalising of self employed single parents who aren't showing a profit, or those not working full time.
However - be very aware that applying for and relying on this financial support can be stressful. I couldn't handle it long term. Getting through to the Tax Credits helpline is almost impossible some days and they can get payments wrong. I had huge problems getting my HB granted, it was a very complicated form and then they suspended it when they reassessed the value of my former home (still jointly owned) I had to complain via my local councillor who held HB in his portfolio and eventually got an apology, reinstatement and even a small amount of compensation for the stress it caused.
For some benefits, your maintenance isn't considered to be income - but watch out for childcare vouchers from your ex - they count as income for Tax Credits/Childcare costs. When my ex was briefly paying me the 30% of the rest of the childcare costs as his sole maintenance to me in vouchers, I was very annoyed to find that Tax Credits wouldn't pay the 70% - but only 70% of the 70%, even though the vouchers came from him (and he received the tax relief, not me!)
Thankfully he changed jobs and started paying it in cash which changed things and they then paid the full 70% to me. It's madness but that's how it worked.