Look at these two:
http://www.landlordlaw.co.uk/landlords/possession-notices/intro
http://www.propertyhawk.co.uk/index.php?page=magazine&id=346
They both have something to say about service. Note that you can send it by ordinary post if you get a certificate of posting, or put it through the door if you have an independent witness who then completes a witness statement.
However, it is probably best in the circumstances if you can catch the tenants while they are in & hand it to them (it must be given to the tenant - or one of them - named on the tenancy). Best to have a witness to this as well. Make sure you keep a copy of the notice.
I assume you have a right under the tenancy agreement to visit and inspect the property. You could contact the tenant without saying anything about the S 21 notice and ask to visit by giving whatever number of days notice the agreement requires, & make sure they'll be in when you go.