Serving Notice - written letter or Section 21

I have recently served notice, 3 months ago, giving 6 months notice which was done in writing, letter format not as a Section 21. The reason for this was I believed from my copy TA it said it was no longer in a fixed Contract but periodic therefore a Section 21 did not need doing.
I have now been informed by my tenant, 3 months later, that allegedly she has been advised that my notice in writing is not valid and that I should have served a Section 21.
Can anyone advise if this the case or would my written notice / letter still stand? Thank you

Unfortunately, the tenant is correct. A landlord can’t serve a s21 notice at all during the fixed term and during a periodic tenancy, it has to be served using the form 6A. If the notice complied exactly with the requirements of form 6A, but did not use the form then it might be arguable, but its not an argument I would like to make to a judge, particularly at a time when they are looking for any reason to dismiss a case quickly and move on to the next.

On the Government website it does say you can serve a S21 written by yourself but it has to contain all the information that a S21 contains.