Random payment from a company I left 2 years ago?

[Justmyluckhey]

Hi, wondering if anyone has had this before?
I worked for a big national company for around 8 months in 2016, left, got final pay (assuming I got it all, don't remember any issues at the time, but it was a stressful time in general and I left suddenly)
So today I received a payment from them into my bank account? Ref is just numbers. It's not a massive amount (well it is for me right now!) But in the general scheme of things, it's not - £50.
Checked with bank, they say it's definitely not their mistake.
Emailed payroll at the company - await reply.
However I'm worried about this on 2 counts -
I'm on tax credits, this is going to push my earnings up (if indeed that's what it is) by £50 and I'm worried that it's going to look like I just haven't told them, and I'll get into trouble. However this company also overpaid me twice when I worked there (paid back) but if I change my TC award, then it turns out to be a mistake to be paid back, I'm down on income, which I can't afford.
Secondly, my account was overdrawn, so even if it was a mistake, it's gone. I'm having some financial problems right now and don't have an authorised overdraft. So even if it is a mistake and needs paying back, it's not as simple as transfer straight back, it'll have to wait until I get paid.
Has anyone had an experience of this before?
Could it feasibly take them 2.5 years to discover they owe me something, just done an audit maybe?
Could it be a tax rebate? Though why not through my current employer? Does that happen?
And why do they still have my bank details 2.5 years after I left? That's confusing me the most? Should they have kept this?

I'm worried because they didn't rush to sort out the 2 overpayments when I worked for them. Didn't respond to my contacts and then took both back with no warning in one month. I don't think they'll rush to sort this out and it's worrying me about the TC thing.

Any help gratefully appreciated!
Thank you.

Someone on payroll has probably clicked on the wrong payee
I did it a few years ago and the only way to get it back was to contact the lady involved and ask her nicely to send it back. The bank can’t do anything
I suppose they could prosecute you to get it back but it might not be worth their while
Of course the right thing is to give it back and as you’ve contacted them it looks like that’s what you are doing

Well yes, if it's not intended for me then I'll pay it back, but I'm a bit annoyed that I'm probably going to end up with some repercussions for someone else's mistake, and also that even though I haven't worked there for 2.5 years they kept my bank details, if they hadn't it couldn't have happened. Only person this is going to inconvenience is me.

Is it WHSmith? If so, the payment is legit

Backdated payrise?

@HermioneWeasley

Yes it is - I'm assuming you've had one as well but know a bit more than I do about it?
They have my mum's address for me as a c/o address and sent my P45 there, but nothing has turned up there for me.

Could it be your pension you had with them? I randomly got money back from a previous employer and it turned out to be the pension money that I paid in. I got the letter explaining this about a week after the money arrived.

Did you work offshore for them or have any sub contracts that came under the 24 month rule? I've had payments in the past upto 2 years after because of the way the tax year ended? Odd payments in March are often about getting books in order.

Surely they shouldn’t have retained your bank details for this length of time. I am pretty sure that breaches current gdpr guidance of only retaining information as long as necessary.

No offshore or sub contractor work, just a run of the mill retail job.
Not sure about pension.
And yes, I really do want to know how they still have my bank details over 2 years after I left.
Thank you for the replies.