When to serve a Section 21 on Housing Benefit Tenancies

I have let my HMSO on individual contracts on tenants who have the bulk of their rent paid by Housing Benefit. while they make a some contribution payment every week. Housing Benefit is paid directly to me every 4 weeks ( in arrears).

My questions are
1) what is the correct way for me to first set up my tenancy agreement ( is it weekly or monthly or 4 weekly ? )

2) If weekly do I have to wait 2 months from the day of the week that the rent is paid ?
3) Would I have to give tenants a rent book ( They already have a signed record of contribution paid.

Thanks

Mr oberai