Section 21

Possession - Section 8 and Section 21

I have a question abot a section 21 notice. The tenn
ancy in question is a periodic tenancy with a rental period of the 20th of each month.
The landlord sent a section 21 notice for possession by post but the tenant claimed not to have received it. The date for possession on the notice expired on the 19th of the month giving 2 months notice.
The landlord then took the section 21 by hand and the tenant signed for it. This date was the 9th of the month so the notice had less than 2 months to run.
What is the position? The landlord claims he posted the notice giving plenty of time. When he subsequently hand deliverd it there is less than 2 months.
Does the landlord need to issue a new Section 21 or can he rely upon the original one but simply "add a month on" to coincide with the next rental period?

Thanks in anticipation.

Peter