requesting money from guarantors

Guarantor

Hello. I need some help / pointing in the right direction. My tenants, who i served S21 notice on 14Oct to be out by 15 Dec as they have always been late and i have always chased payment ( sometimes 6 weeks late). Their STA ended 15 Nov. Anyway they new they were being served notice as they were behind with payments and on 14th Oct paid up so only owed me £150, monthly rent in London £950. On 15 Oct ( Their rent day) they paid £500, and nothing since. They now owe over £1500 and i want to write to their guarantor - His father. Is their a polite forst letter template - I want to be polite and firm without being threatening.I do not have the guarantor phone number, but do have address, work address etc as they were credit checked as a guarantor. Has anyone been in this situation and offer any input. Thnx in advance