Question About Experian Entry

Good Morning

I have been working hard to improve my credit score following a period of financial difficulty, with a view to getting a mortgage in the next few years.
I cleared most of my old debts and set up a payment plan for a defaulted account with Welsh Water. I owed them £900 and agreed a monthly direct debit amount that would partly cover the default amount and the rest to go towards current bills.
This has been paid regularly for the past 6 months or so, however I checked Experian today and it shows me as being 1 month behind on my payments - I called Welsh Water who said this is correct as the amount I am paying is covering the arrears so the 'months late' will continue to increase on Experian until the arrears are cleared and I am paying off current bills, as I am paying £80 per month this will take forever, given the debt increases by around £50 each month as bills are produced.
Short of clearing the full debt (which I am not in a position to do) is there anything I can do as this has now affected my credit score that I had worked so hard to increase! Should they not have closed the defaulted account?

Thanks

Gab