Working our interest owed

Possession - Section 8 and Section 21

I finally (fingers crossed) have a possession hearing under section 8 next week. Tenant owes 9 months of rent payments and has not provided any explanation/reason for not paying. As yet, no defence has been issued to my claim either - I believe the tenant is simply waiting to be forced to leave - when bailiffs ultimately are instructed.

Under the terms of my tenancy arrangement, the tenants are responsible for interest at 4% above base rate calculated on a daily basis. Given the sums of money owed, without interest being applied is now several thousand pounds, has anyone bothered to add this to their claim before? If so, how did you calculate it?

Is it simply (interest rate) / 365 days x arrears amount?

Do you then work out each month in turn across the nine missed payments?

I wasnt sure if I needed to know this answer at this point or whether this needs to be understood if I make a moneyclaim for the unpaid rent in future?

Any help/guidance greatly appreciated.