The first thing to note is that you can’t make it as a loan because (a) you’re not lending them any money and (b) you’re likely not licensed by the Financial Conduct Authority. Therefore, any references are just for an amount owed as unpaid rent.

Because the Tenant Fees Act now applies to all tenancies (including existing terms), the maximum you can charge is 3% above base. This would literally take forever to go through each month, work out how much was due in that month, apply interest etc. and all for a few pence. We would just take the final amount now due and start calculating interest for the whole amount as from the last day of the tenancy.

Please see here for the Tenant Fees Act and the interest that can be charged.

A repayment agreement is not something we have currently but plan to add something maybe even this week (but not sure yet). It doesn’t have to be complicated though and can just be done by email. It will simply confirm the final amount owing and the agreed payment amount per month. If they reply confirming the agreement that will be great but just paying the first instalment will be sufficient to confirm the agreement.

For example if the amount due is £1500 with a suggested payment plan of £130 per month, you would put something like (ignoring interest because it’s too complicated for this reply):

We confirm the balance due of unpaid rent for [tenancy address] is £1,500.00.

We confirm it is agreed that you will pay the following amounts on the following dates:

06/01/2021 – £100
06/02/2021 – £100
06/03/2021 – £130

And, thereafter on the 6th day of every calendar month at the rate of £130 for a further 9 months until the amount due is cleared.