HMRC still want £416 off me

Over the last few Saturdays I've detailed my struggles with trying to get the government to grasp that I have brain damage, memory difficulties and depression, and that these are having certain lifelong effects on me.

HMRC have looked into last year's tax credits award and have decided that I owe them £416. After tax, this is about half my monthly wage. It's not a small sum to me. I've asked them to look into this again, and they are sticking to their story.

Added to this, I'm finding conflicting stories as to whether I'm eligible for tax credits at the moment. ATOS only awarded me the Mobility element of PIP and not the Daily Living component. I understand it's this latter element that would allow me to claim tax credits if I work less than 25 hours a week (I'm on 22.5).

After a meeting with my manager, and a look into the tax credits calculator, it appears like I should be eligible for a certain amount. By the 5^th April, it appeared, I should have heard back from HMRC.

Well, I have. They still want the £416. Apparently I forgot to reply to a letter from 25^th January last year, a 'TC818A' (I have no recollection of this, but hold in mind the condition that makes me eligible for benefits in the first place).

What a shocking state of affairs. I phoned HMRC numerous times and not once did they mention this form. I would have written it down and acted on it. So, I now have to pay this 3-figure sum back, at a time when I'm making my wage, plus the smaller mobility element of PIP. Before PIP was introduced and my DLA was stopped, I was on both elements of DLA, and WTC with the DLA element. I was eligible for free prescriptions and dental treatment. Now I'm not eligible for anything other than the element of PIP.

This is happening in 2018.

I think there may be other letters that I've misplaced or not acted upon, and this is an issue I've had my whole life. I can't keep up with the stream of formal letters that, largely, don't make any sense to me. I've applied for assistance from a few departments in relation to this, but I expect that because I work in local government and I speak well, and because I've never been arrested, that application has probably been ignored.

Things are up and down in work at the moment but on Tuesday I'll update my manager and see if he can make sense of this. If you think you can advise (layman's terms, please) tweet me.