Tenant has been given section 21, they are now asking to move out at the end of the month instead of the 18th (date on section 21) if I let the tenant stay till then, will that make my section 21 notice invalid and have to start a new one or not?
No, it wont. The s21 doesnt end the tenancy it just allows you to apply for a court order, if they dont leave, which you can do up to 4 months after the notice ends.