Apologies for the long post

Our previous landlord is attempting to make us pay a extra weeks rent due to a key being returned a few days late here is a quick rundown if the situation:

Our tenancy ended on the 3rd of February, we had our final inspection and no issues came up, signed the bond return papers returned the keys (we were given two house keys originally but had one privately cut) and vacated the premises leaving it in a clean and tidy manner, one flatmate was away so couldn't return the key till 4 days later which we were very clear about with the landlord, a week later we received a email from the landlord stating that as one key was not returned we owed another weeks rent, I had a few phonecalls with the tribunal and they told us that he was being unreasonable and to submit the bond return forms ourselves, we then recieved a email from the landlord claiming what we did (the form) was illegal and that he called the tribunal and they had zero record of conversation between us (I honest doubt they would disclose this information) and to pay less than a weeks rent directly into his bank account and he will send through the bond papers to avoid the hassle, I have attached screenshots of his emails I left out my replies as there would be to many screenshots but they were basically just what the tenancy services told us etc, this situation is understandably quite stressful for a group of students so am looking for some advice for the time being

Edit: we were also never given personal copies of the lease after it was originally signed

Is our previous landlord being unreasonable?