Landlord trying to charge me 25 quid for an ‘invoice’

I feel like I’m going slightly mad with this so would appreciate some outside opinions.

I moved into my flat recently and my landlord has charged me £50 for apparently “misusing” the communal bins.

They’ve confirmed in writing that the £50 is:
£25 for breaching my tenancy by misusing the bins
£25 for sending me the invoice

And when I say “invoice”, it’s basically an email saying I’ve breached my tenancy and I owe £25 for the breach + £25 for them sending me that email to tell me. There’s no actual breakdown of any costs.

I genuinely don’t know what I’ve supposedly done to misuse the bins either. They’re communal bins for the building, they weren’t overflowing and the rubbish wasn’t all from my flat. I had just moved in so obviously there may have been more cardboard than usual from unpacking, but nothing was overflowing.

I checked my tenancy agreement and can’t find anything about the charges for bins.

So I emailed them disputing it. I specifically asked them to tell me what the actual damage/financial loss was from this alleged breach and to give me an itemised breakdown showing how they’ve arrived at £25.

I also told them they can’t just charge me a £25 admin fee for sending an invoice under the Tenant Fees Act, so that part should be removed as it’s part of tenancy management.

They haven’t answered any of it. They’ve just come back saying they won’t waive the £50.

I’m obviously still paying my rent as normal. I’m just refusing to accept a random £50 charge without them actually explaining what it’s for. Not to mention the many maintenance repairs Ive put in that have not been addressed.

I’m tempted to put in a formal complaint now because I don’t know how else I’m supposed to dispute something when they won’t actually answer the questions I’m asking. Where do I go from here?