Section 21 served on tenants

Possession - Section 8 and Section 21

Last November I sent my tenants of 9 years a section 21 through legal4Landlords. At that time they were very much behind in their payments. they were pleading to let them stay and promised to pay on time etc. although they are only £115.00 behind now, they never pay on time and are very difficult tenants. there are arrears from previous years which I have to deal with later. When they pleaded I let them stay on until the end of the contrat which is 15th July. Now they have sent me an e mail asking me to explain why I am threatening them with eviction again. I was of the understanding I dont have to give an explanation for section 21. i dont like to reply because last year they were giving me a lot of stress saying I have threatened them etc. I understand that councils encourage them to state things like that to take land lords to court. I have also heard that councils ask them not to leave as then they are told they have made themselves homeless. i understand that councils want an eviction order from court in order to house them. Am I right?. am I legally bound to reply to their latest e mail. After all i allowed them to stay another 6 months after the last section 21. As they semm to be reluctant to leave can I take court action on the grounds that a S21 was served last November. I thought thats nor right as I let them stay until july. Should I stay until 15 July and then take court action or can i do it now as Legal for landLords suggested before I asked them to send the econd S21? A reply will be very much appreciated and "thank you" Rainbows