HMRC help, reassurance, gin?

[Cameron2012]

I am having a bit of a mare with HMRC.
In 2016 I changed jobs and also set up my own little business.
I registered as self employed and in Dec filled in self assessment.I also had to declare earnings from previous job and new job.
Bill came through and realised it as incorrect, only money owed was small payment of £80 from self employment not £1200 HMRC said I owed.
Sent letter with copy of P60 showing correct PAYE had been paid and my calculation of tax owed.( £80)
Rang them a week later and they said they agreed to within a penny of my calculation.
They sent me a new tax assessment for £80 and I paid my tax.
And that should have been that.
This week I received a letter from them saying I owed them £1120.
I rang them and spoke to rather unhelpful lady who put me on hold while she read my notes and then rang her colleague to discuss it.
Then she came back on the phone and said they would look into it and would ring me Within a couple of days.
She was frankly dismissive, I said that I was concerned that legal action could be take against me and she said that that wouldn’t happen she would put it on hold for a couple of weeks.
I have had no phone all back and today received a letter dated 8th March saying I owed £1120.
If you have read this far , thankyou😬
I will ring them next week but am in such a state, has anyone else gone through this.
I have been honest and paid my tax but have a horrible feeling I am caught in the system

If you completed your SA return correctly and it showed tax due of £80 then they shouldn't be wanting £1120. Are you sure it's not tax due from a prior year ?

Thankyou for replying.
No they agreed the discrepancy was their fault ( they added a tax rebate onto tax paid on the initial assessment),
The woman I spoke to said she couldn’t understand why it was still on the system.
For their initial calculation to be correct I would have had to have earned about £40k, which I defiantly didn’t .Not even close .

Have you sorted this out now ?

Not really able to help much, but I had problem with them last year, they get sending me late filing penalties even though I filed on time. They agreed they were in the wrong and then continued to send me notices. I just have to keep on top of them with it and eventually they rectified it all.

Just remember to make notes of all phonecalls with them, names of people you speak to etc, you will get there eventually

The letters/demands they send are often a few weeks delayed so they may have been created 2/3/4 weeks ago, but only dispatched more recently. So letters recently received are often out of date and don't reflect more recent changes. HMRC systems are slow and cumbersome - it really can take a few weeks between one of their call centre workers changing something and that change filtering through all their different systems. In HMRC-land a week or two is nothing - they simply don't have "live" systems that make changes throughout immediately.