need some advice on whether this would be classed as serious misconduct

[anonymousposter]

I received a letter in feb from tax credits saying they believe another adult lives in the house (there is another adult here, they know this, I'm not debating that point with them) and which advised me to call.

I spoke to someone there, and one of the questions I was asked was regarding whether I held any joint accounts with this person. They then said "oh we can see your name is not on any of (other adults) x amount of bank accounts"

I believe this is a breach of data protection.

I have written a letter of complaint to HMRC with regards to how my case has been handled, the misinformation I've been given by various departments, the fact they have lost sensitive and confidential paperwork belonging to me and also this apparent breach of data protection.

I have two options where I can send this letter.
One is to the department that is handling my case,
the other is to the department that deals with serious misconduct

from their website:

Serious misconduct includes:

assault leading to death or serious injury
corruption
fraud
unauthorised disclosure of customer information

So which department should I send the complaint to?

What is the breach of data protection? I don't see one. Your name is not on the other person's bank accounts. You knew that. So no breach there. The only possible breach I can see is that they have told you how many bank accounts the other person holds. However, as they haven't told you any details of these accounts (other than the fact you aren't named on them) it is hardly a serious data protection issue.