As your tenancy started before Oct 1 2015 the only thing that will invalidate the section 21 is not protecting your deposit.
Gas safety certs, EPCs and How to rent will only invalidate a s21 if your tenancy started after that date but as you mentioned that you have never renewed it then it hasn't. You will now be on a periodic rolling tenancy.
As PP's have pointed out the S21 eviction process can take 6-7 months as you do not have to leave on the expiry of the notice.
As you are no longer in a fixed term tenancy then the landlord can serve s21 at any time and give you 2 months notice.
He would then have to apply to court for the possession order and you would be able to submit your defence that he didn't protect your deposit. He would then have to return your deposit before issuing a new s21 (and therefore resetting the clock).
He still cannot apply to court for a possession order until the date on the new s21 has expired.
Once the possession order is granted - you still don't have to leave by the date on that order.
He would then have to apply to court for a bailiffs warrant. You are legally entitled to stay in the property until the bailiffs turn up.(not saying that you want to do that btw)
When you want to leave make sure you end your tenancy correctly otherwise you will be liable for rent until this happens.
ou must give the correct date for when your notice period ends. Your notice letter won't be valid otherwise.
Your tenancy agreement should set out the amount of notice you must give.
If your tenancy agreement doesn't set out a notice period, you must give your landlord at least 1 month's notice for a monthly tenancy and must end on the first day or last day of your tenancy period.
Regarding the works the LL is doing- you do not have to consent to let him in to do them.