Section 21 and deposit protection

[Langanby]

I forgot to protect the deposit of a tenant within the 30 days limit. I then basically owned up to them and returned it via a bank transfer referencing that it was a deposit repayment. It was only £200. I have read conflicting things about whether I can issue a section 21 notice to evict. One group says they can say the failure to protect it in the first place means that I can't issue a section 21 which seems to infer that I can never remove them if they don't leave voluntarily, which seems ridiculous. To get out of this situation they would need to take me to court and get between 1-3x the deposit. Not great but better than effectively giving them the house. Another says that the return of the money allows for th section 21 to be issued, however, they could still claim the 1-3x deposit as above. Please advise