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Rent Arrears

Hi my tenant did not pay his £580 rent on 27th July. I have been given excuse after excuse and eventually on 20th Aug £250 of the rent owed was paid. Therefore he still owes £330 of arrears and another £580 which was due today. He has sent a text with a proposed payment plan of 3 x monthly instalments of £110 on top of usual rent equalling £690 on 30th Aug and the same amount on 27/09 and 27/10. He has asked me to confirm in writing whether i accept this. I have replied to his text stating that i do accept the proposal and that i will confirm in writing. Is there a particular format i have to follow in replying? Does this affect any future actions i may need to take, such as issuing notice. if i do need to issue notice, would it be a section 8 due to rent arrears?