How do I make a complaint against the FIU ?

Background :- In my CM case the FIU conducted an investigation in to my financial affairs, and they subsequently made decisions which were not in my favour. These decisions turned out to be effectively 'unchallengeable', at least internally within the CMS, as I was told that the FIU have to approve any changes to their own decisions. I was also told that subsequent to the CMS' implementation of the FIU's recommendation the only option left open for me to challenge was via appeal to HMCTS, which I duly did. The tribunal also made decisions that were not in my favour.

In the CMS's submission to the tribunal they duly refer the tribunal to the FIU report (which they included in their submission, and receipt of their submission was the first time I had seen the FIU report) - so the FIU report essentially underpins not only the CMS' decision but also the tribunal's decision.

Now of course it is my responsibility to check the FIU's report for errors but I failed to notice some errors prior to the tribunal hearing. One error was that the FIU looked at the wrong box on my tax return to ascertain my income from a property that I let out (they used my property's gross rental receipts and not the profit i.e. after expenses were accounted for) - this error caused them to use a figure of £18k as my property income when in fact it was only £2k. They then used this incorrect property income figure to overestimate my projected income in retirement and on the back of this they ruled that my additional pension contributions were excessive and thus amounted to 'unreasonable diversion of income' which should be assessed. Probably went in to too much detail there but anyway it was a silly error they made. There was another error which I won't bother going in to but again it was relevant to the decisions at hand.

Anyway, so now the Lower Tribunal have made decisions not in my favour and I have only just realised the errors above. It seems that I am now faced with another lengthy process of appeal to the Upper Tribunal if I wish to have the decisions looked at again in light of these errors I mention.

So I wrote a letter of complaint about the FIU to the CMS, describing the errors made and my situation as I describe above. I have today received a reply which states :- "'We are unable to investigate your complaint against the FIU. They make their findings and we just implement them. We cannot comment on or query them".

What ??!! How can the FIU be so 'untouchable', even when they make elementary mistakes which can be clearly demonstrated ? This does not sound at all right.