4 weeks in areas

Rent Arrears

Hello there
I signed a One Year tenancy agreement on 6th July 08 for monthly payment of agreed rent. A month later the tenant rang me and ask to pay weekly. I agreed but the changes were not put in writing. In Sept the tenant missed 3 weeks rent payments and then started paying again without topping up the missing payments. Last week again I did not receive the weekly rent so - 4 weeks in arrears already. Do I need to wait until the tenant reaches the stage of 2 months arrears before I start any actions?
The tenant is a student but her mother signed as a Guarantor and Rentguard checked them both. In August during a telephone conversation the tenant and her mother reassured me that the rent would be paid soon. It’s the end of Oct now and I'm uncertain how this tenancy will proceed. I don’t have Landlord insurance but if I take one including Legal Cost / Rent Loss now, will it protect me for the rest of the period? Please advice if there is anything I could do at this stage? Many thanks. Dan