[Solved] Untold Arrears

Hi guys, first post here as i've been desperately trawling the net looking for answers.
Its regarding arrears from 2011 for which i was never told about until about 3 weeks ago.

I split with my ex back in March 2011. I used the online calc to offer her maintanance, but like most, she thought she could get more out of me and eventually went through CSA in August of that year.
Anyway, they set the amount to pay at £305.
At the time i had one of those crappy bank accounts that was like a pre-paid card and my direct debit for the payments kept failing, and i had to call them every month for about 4 months, to pay it on the card.
Eventually, without warning they just wrote and told me they would take it as a DOE. I wasn't too bothered as it made life easier.
Anyway, they carried on taking out of my wages, until i eventually left that job at the end of november 2010.
I had very small 1% annual payrises but not much of significances.
I then spent 6 months on Jobseekers, again they took it straight out of my payments at £5/week.
When i started a new job in May 2011, my ex said that she no longer wanted to use the CSA as she didn't want to wait 2 weeks for the money to get to her. (A perfect example of her illogical thinking as she still got it every months regardless of whether it was 2 weeks after i actually paid it!)
So i agreed and as my new wage was a little lower, and the calculator gave a figure of about £295, i just rounded it off to £300 which she agreed to.
Only now i realise why she was so easy about it when everything she had done in the years previous was to make my life as difficult as possible.
This is because my new partner was pregnant, and at the time, i didn't know my payments should reduce with the birth of another child. I only found that out in the last 2 years or so.
But anyway, even with that, i carried on paying £300. My wages slowly crept up with annual payrises, but nothing major, but i also had a second child with my current partner.
Anyway, i left that job in 2017 for a much lower paid job, and low and behold, she wanted to use the CMS again.

At this point, i had never missed a payment, never recieved anything from the CSA, nor any objections from my ex with regards to amounts being paid etc.
But my question relates to the original CSA agreement from Aug 2009 to May 2011.
Recently, i've had a letter saying i owe £1500 arrears!
I said that i'm not paying it without evidence and i dont see why i should prove that i DONT owe something when they haven't proven that i actually owe it.
So they sent me a statement for the perios in question.
I've noticed it is littered with increases and adjustments that nobody had ever told me about.
There were increases from £70/week up to £90/week in july 2010 despite no payrise backing up that rise. There were also several adjustments of hundreds of pounds, (highest was £930) and some were recredited, others weren't.
There was also and £1800 credited back, because they carried on totting up the debt, from june 2011 whilst i was paying her direct, until feb 2012. Clearly at £300/month, that £1800 rebate was not enough.
There are several things to this story:
Firstly, the arrears were built up through unwarranted increases for no reason.
Secondly, if the increases were valid, is it fair that they didnt advise me of them?
And even if they claim they did, i was paying through a DOE, which they had control over, so why wouldn't they have simply increased the deductions?
and last but not least, is it fair they can leave it and say nothing for 7 years before writing to tell me i owe it?

Also, is there anything i can do about paying too much when my other children were born, as i have only recently found out i could have been paying less? Despite being in a private arrangement at the time.

Being such a long time ago, its proving difficult getting all the relevant evidence together so i wonder if there was anything i could do.
MY current partner doesn't work for valid reasons so we do get benefits to top up my wage and i rely on that to help with bills as i only have about £30/month to live off from my wages after bills, but if they are going to chase this, then i will be better off quitting my career and letting my partner go back to work instead. We'd be worse off but i'd be saving almost £300/month without the CMS
Thanks in advance