Over payment from a few years back. HMRC at fault but still trying to claim money back from me

Danboy1973

I had a letter in March saying I owed over 5k from 2017-18 and 2018-19
My wife is SE and I have a pension.
It turns out that 17/18 that tax credits made a mistake and put my pension down as a PAYE and again as a pension on the same year so this doubled that years figures.
As for 18-19 when I rang up with the SE figure from that year they put something completely different down.  we never really noticed as our payments didn't really change as we get max TC anyway as low income where we starting a business. 
In March after receiving the letter I straight away filled in the dispute form.  Last week I got a letter saying 'that because I never responded to a letter back in April 2020 regarding the over payment the 3 months to dispute it are now up'.  I never received a letter back in April last year, I was lucky to even get any post because of the first lockdown.
The letter I got in March this year was the first I knew about it, it also didn't say anything about a previous letter being sent.

Surely there must be something that I can do but I don't know where to start, im now on UC as the business was affected by the virus and now TC are going to start taking the money owed from that.   

Alice_Holt

Maybe try the statutory official error provisions, but dealing with HMRC on previous year disputes is very difficult.
(It is really important to check that the annual returns are correct at the time).

Some info here:
https://revenuebenefits.org.uk/tax-credits/guidance/how-to-deal-with-hmrc/disputes/

A subject access request make help you.