Informal Agreement vs CSA

RP OH to save confusion:

I have paid my stbx £500/mo for 3 children all under the age of 12 since our seperation in May 2008. This has never changed , even thru contact being denied recently, overnights being revoked etc due to false allegations.
In fact I handed over £950/mo up until August 2009 to cover household costs based on half of mortgages, loans, and various other payments, this was more of a demand and division of financials were never discussed, I did this in good faith.
This left me with £300/mo for bills, rent, and all living costs. Thankfully my amazing partner was on a very liberal wage and since she already had a flat I moved in with her in April 2009 and contributed all my leftover pay to her, which certainly was not near 50% of household costs but she never questioned this.

In late July 2009 she was made redundant and had to find new employment, which resulted in a pay cut.
We decided to comb through both of our finances, including doing checks on what my stbx said I was 'liable for' monthly.
Turns out I was actually paying 100% of the mortgage on two houses (i only) two full loan payments and payments on household goods loans in full.
Basically the only things she had to find money for was household bills and council tax.
She was and is still unemployed, her choice. She continued to collect wtc until late 2009 (which I am now repaying 50% of back to HMRC even tho this went into her sole account) and ctc, and in addition moved 3 mates into her downstairs reception rooms to accrue rental income in the FMH.

All that aside, I requested a formal breakdown from her in August 2009 for my informal SM payments, to which no response was given. I ceased paying this September 2009 and now only pay the informal CM. Any loans that were in my name I transferred to my sole account to protect my credit rating and as due to her income and knowing she had the ability to pay the mortgages I stopped contributing.

After putting a lot of this on paper I realised that she is making quite a hefty income, and more recently is now cohabiting with a new partner who may or may not be contributing to the household, and she still retains rental income.
I do realise the cost of children is great. £500 does not go far and they most likely eat £300 of that in food shopping a month. I have never really had a problem with the amount I pay for CM, it does get very very tough towards the end of the month, saving anything is out of the question. But when the children are with me, they come dressed in rags, shoes with holes in them. I never question this and if there is an immediate need for clothes, shoes etc I purchase these for them and send them back home, altho I do never see these items again!
All the while this is going on my stbx is spending like mad for herself, new shoes, new car, new everything.

Based on the CSA calculator I should be paying 325 a month if they are less than 52 nights overnight and quite frankly if it dropped down to this the additional 175 would go straight on the childrens backs or in their mouths when they are with me, or be spent on outings with them etc; which at the moment are tough due to finances.
My stbx is a very stubborn, manipulative woman. I know that if I approached her with this figure she would spit in my face.
BUT if I approached the CSA directly, and elected to pay via them, there is a chance she would revoke contact. Having just had our contact case adjourned, I am hesitant to go this route. That being said, it may force her into engaging into discussions about finances, and we may be able to agree a way forward in that regard, as I am about to Petition for divorce.
I know that she will not want to go to FDR and will want to keep everything out of court if she can. She makes a tidy rental income in our 6 bedroom house that she does not declare, however if she doesnt declare it a judge may deem it not suitable for a family of 4 and order it sold, so she is in a bit of a catch 22 herself. We also have another property that is tenanted at the moment however there is a bit of equity in it.
Having spoken to a sol about this I know I am in a good position in regards to financial/property settlement in the divorce as stbx will want to settle cleanly without form E's ect. My main concern is if I reduce CM, I will no longer see my children. She is not shy of a false allegation but I want to be able to support my children as best I can..
Basically just looking for some thoughts on this one.

if you reduce cm (which I think you should she is clearly raking it in and you are sturggling) then yes she can reduce contact but then you can apply to the courts as it will be quite clear that you have a pattern of contact and that this needs to remain in place.

When you do reduce it just expect a kick off or some reaction and then try to roll with it as best you can.

My rule pay the minimum you can to her and then allow the money you have left over to treat the kids when they are with you.

Thanks for your thoughts GA.

After speaking to a few people about this I have decided to hold off for 3 months, to ensure that there is a clear pattern of contact, obviously keeping diaries etc so if any allegations do arise then I have something substantial in my favour.
I am also aware that my stbx was not impressed with the fact I applied to the courts after contact broke down last year and if it were to do so again she would find herself back in front of a judge, which does revoke her 'control', something she holds very very dear.
I too think that I should pay the minimum, but I would like to be able to contribute to school fees, uniforms as these arise, on top of the CM rather than pay over the odds in good faith these things are being included. I think its good to communicate about the children as much as possible and all the little monetary necessities that come up are a good place to facilitate that communication in addition to everything else.

I would much rather my children are taken care of than hear about my stbx's growing handbag collection!