The 1 month notice applies to tenancy agreements NOT renewed and going on a statutory periodic tenancy after the fixed term expires. if they sign a renewal, they are bound by the new fixed term and so are you.
Which makes me ask the question. Why do they want to renew? Why do you want to renew?
As for the guarantors agreement, it depends on what it said. If it covers renewals and periodic tenancies, then you are OK. if it is referring to the specific tenancy signed 6 months ago, you might need a new one. Not sure why you'd ask for new ids or new references.
All views posted reflect my personal opinion only and do not constitute professional advice which I am not qualified or knowledgeable enough to provide.