Firstly, have you checked if you're entitled to any benefits or extra support? www.entitledto.co.uk/
I sold on Vinted during mat leave, mostly baby clothes as my daughter outgrew them. It doesn't make a lot of money but it's an extra little boost. You should definitely look into this, the app is very ease to use.
Another thing I tried was matched betting and I made quite a bit from that (over a grand) in a short space of time. You need money to get set up with that though, but it's risk-free, it's not actually gambling. You need to read the tutorials though and really get your head around it though, but once you've done it a couple times you quickly get the swing of things. I'm not sure what offers there are on at the moment, so hopefully they haven't dried up, but I did it a year ago.
I use the website Topcashback for all online purchases. I've made hundreds on this, all the pennies and pounds here and there add up over time
There's also a £10 a day thread over on the Money Matters board. I dabbled in a bit of this to put away some extra money before going on maternity leave, but it was really slow going. Mostly surveys and things like prolific.
Are you artsy or creative at all? You could consider creating handmade items on Etsy
Also, when I was on the unpaid portion of my maternity leave I picked up work through an agency working in a warehouse. Even just 1 or 2 days a week adds extra to the family budget.
Have you got any KIT days you can make use of at work as well?