Stop giving your SO so much money. Calculate what you and baby take up food, utility, and housing-wise and give him that. If he balks, tell him you don't have the funds to continue to subsidize him and his children since you're responsible for 90% of baby's things.

Then, stop doing all the chores. Wash your dishes, put away your clothes, etc. If your SO wants to act like it's not also your house, then behave like a roommate and stick to your own areas and keeping those clean.

Also, stop lying to your SO about why you're going to bed early. They will either step up or leave you. You already said it would be easier by yourself, so the threat of leaving isn't much of a threat. It may hurt, but it's not going to kill you. 

Don't stay in a relationship that tears you down. You should be better off in a relationship, not worse.

Frankly, I would not be happy with a man who was more than happy with this situation, and enabling his adult children. If he likes their company so much that he is willing to forgo time alone with you, let him have them.

My life would be so much easier if it was just my son and myself.

Then do it. Make the decision and do what is necessary to make it happen. Stop letting them taking you for granted.

F that.

Id leave, quite frankly.

You've said life would be so much easier if it was just you and your son. So do it.

What adult child wants to go to the pub with their Dad and his partner all the time???

Yeah that's a huge nope! There's no way I'm cleaning up after grown people! Take their messes to their rooms. Get a small container with a lock to store your baby's snacks. Break the household bills down to where you pay 30% of everything. (100% divdided by 5 people=20%, plus 10% which is half of your child. Your DH should be paying the rest). If DH continues not to help with Child costs, start taking half of that from what you pay. He will only use you as much as you allow. in the meantime, start putting the money you are saving into YOUR account (not a joint one) in case things go bad and you need (or choose) to get out! 

When she is asked, she'll go to her mums for the night so that she doesn't have to.

Ask her to do things more often. Maybe she'll get tired of changing houses and move to BM's permanently.

The entire money situation is unfair to you. SO should be paying 100% of his, SS's, and SS's share plus 50% of your joint child's. You pay your share and 50% of you child's. You each pay your individual expenses. Stop buying everyone's toiletries and the cleaning products. Buy your own toiletries and lock them up. Cleaning supplies come out of household money not your personal money. Don't buy food for everyone. 

My life would be so much easier if it was just my son and myself.

If SO doesn't change, make it so.

Your baby's food and this needs to stop immediately. Your husband should be contributing towards baby items not you!!

Tell him you are fed up of 2 lazy grown arse adults freeloading at home whilst you both work hard

"What do I do?"

I have a couple of suggestions:

1. Stop paying for anything for them (this includes your husband). No more toiletries or food for them. They want something? They buy it. And stop cleaning up after them, cooking, doing laundry or anything else you do for them. Not your problem. You are simply one of the inhabitants of the house and as such will only take care of yourself unless everyone else mucks in.
    
2. Find yourself a nice small appartment for you and your son and move out. No need to divorce, just live separately. Of course, your husband might choose to move in with you - as long as it's clear you're not going to go back to his current home and that his kids will not be treating your home like a hotel.