Does s21 apply?

I have a tenant who has been in place since July 2004, on a weekly tenancy. This obviously matured into a periodic tenancy.

I am selling the property, so wrote to them giving them advance notice that it would be marketed - and that I would be grateful if they would allow viewings.

I wrote again last month with a formal notice to quit - but NOT in s21 format. I gave them two months notice.

TODAY, I had a call from the Council. Apparently I must use s21 notice.

BUT - the form 6a clearly states on P1 that "there is no obligation to use this form in relation to ASTs created prior to 1 October 2005"

The council seem determined that I must use that form, and they seem delighted that the delay may stop the sale (I bet they advised the tenant to remain until court order, too).

What is the correct approach??!!