Just received this through my door. Any advice please??

Slinky

I'm guessing from this that the discrepancy has occured between the small differences between the weekly and monthly rent payments, totted up over 5 years.  Your monthly payments have been assigned to your account and gone mostly against the rent, rather than the water figure. They could just as easily paid the water off in full and the debt would be against the rent. I don't think this is actually an issue about an outstanding water charge at all. It's the discrepancy between weekly rent and a rounded down monthly payment.

Soussman

One thing I haven't seen anybody say in their replies is that if the tenant was told they had to pay £455 per month rent and £20 towards the water rates, the total being £475, surely because they were told that and they've paid that every month as agreed then the OP shouldn't be liable for anything else they're told they owe surely? Because they've always paid what they were told to pay by all accounts.

And to threaten them with taking possession of the property for the sake of £97 is a massive over reaction surely....