A bit of a rant I'm afraid...
Thank you for all your advice and opinions. I do appreciate your support and understanding.
Luckily I do not have to pay back any additional maternity pay. 
My employer doesn't do part time hours. They turn down all requests for returning mothers. I work for a huge French it company who undertake the dwp assessments to see if people are fit to work. They are as awful to their staff as those they assess. My boss was a bit of a git so I needed to leave anyway.
To those who criticised me, please can I give you some more info before you judge me so very quickly.
I receive the salary I do after 9 years with the same employer and yearly pay rises. Those pay rises ended when I was made redundant in 2010. I then took my current job which at the time was ten miles from home. My employer was then acquired by another bigger company and we were all moved to reading, an hours drive away. A year ago I was moved to London so I did not choose to work in London. It was there or no job. It would take me over two hours to get to work from here. My boss would not let me work from home when I had severe spd so do not expect him to let me change my hours, not that this would help me anyway. They also will not be giving me a pay rise any time soon as I earn more than my peers due to me coming from another company. (They have told me this).
If I moved closer to work and was on my full salary I don't think I would qualify for housing benefit would I? Without housing benefit I don't know how I could afford all the rent and all the bills in London. Plus I don't want to be cut off from everyone I know.
As for my ex, he was emotionally and physically abusive and his behaviour got worse when I was pregnant. I finally got the strength to leave him after posting on here - everyone told me to leave him. He has just been made redundant so I have not been receiving anything through the CSA from him. He already has two children with his ex wife so the CSA contribution is £47 a week. For info the CSA has only just been set up and isn't due to start until 6th April.
With regards to keeping my current job and commuting -
My monthly take home was around 1775
Travel from here to London is 523 a month.
My rent is 825 for a really small two bed in a run down house (it's the cheapest thing I could find in my town and I need cheap due to the lha limits).
So that leaves me with just over £400 to pay all my bills (council tax is £100 a month alone), buy food, nappies, milk, parking at the station, run a car (I'm in a rural location so need a car) etc etc.
I don't know how I'd pay for nursery fees on top!
I have no family to ask to help with child care as mum isn't mobile enough to look after my ds and has severe depression. My aunt lives in Devon and uncle in Canada.
Moving closer to work isn't feasible as I need to be close to mum and ailing grand parents.
And before someone says I shouldn't have had my ds if I couldn't afford him, I could afford him before my ex went all crazy on me and flipped out.
As soon as he starts at school I will of course seek full time employment. I trained as a Montessori nursery teacher and would love to go back to that one day. I hate my current job as a pa so a career change is very welcome. I am going to look into becoming a virtual pa which I can do from home. If not this, I'll find something else part time. I'm also considering going back to uni at some point so I can do something more worthwhile career wise.
I think bringing up a child is the most important job in the world and I didn't go though everything to dump my ds
on someone who couldn't care less about him and is only doing so for a wage. I know that not all nursery workers are like this, but some are. I know as I've worked with them.
For those who are offended, ask yourselves, if the shoe was on the other foot, if you were me, what would you do? Stay at home and bring up your baby yourself or leave home at god knows when, get the train at 7am, get home past 8pm, collect your baby from whoever has him just in time to put them to bed? What's the point? I might as well put him up for adoption now.
If i put him in childcare i wouldn't get to hear what his first word was as no one would be paying him enough attention to notice or care. Can you honestly tell me hand on heart you would rather slog your guts out and never see your children awake than claim benefits for a few years?
I'm sorry about the rant, this has been a huge decision for me. I'm not a scummy benefit scrounger, just at the moment it's the only thing that makes sense. Even the lady at the citizens advice told me it was my best option.