Any question answered...

Is it a standing order or direct debit?

If it happens to have been a direct debit (unlikely in the case of private landlords) then you can just get the bank to refund you (as part of the direct debit guarantee makes the bank liable for any mistakes)

Sadly not a direct debit.

An avenue worth exploring then. See if you can find/obtain any logs of online activity. Might be able to see where the process broke down. If it's a bank error then I can guarantee that payment will be reclaimed in no time. They're just being lazy.

Online banking were all shrug we can't do anything.

Don't rely on a text - send a letter, recorded delivery. Give him 14 days, then take him to court.

Cheers, agreed, hopefully it won't come to this though.