I am wondering if anyone has dealt with an unfair default from Anglian Water?

My partner has managed to build a brilliant credit score, after some young adult mistakes effecting his score. He checks it weekly due to currently attempting to buy a house and for his business.
This month a default has appeared from Anglian Water stating from 2018 and for an address he didnt live at full time (My address). He wasn't registered at this address in 2018 or 2019 yet apparently was put straight onto my Anglian Water bill Soley in his name.

When I moved into this address I didn't recieve any letters in my name or my partners name. The only letters I recieved where addressed to another person and with an incorrect post code. I gave several of these letters back to the postman in the hope they would reach the correct person. During this time Anglian water never contacted me and never came for a water reading, and in all honesty when I purchased the house i was told i was on a private sewarge system, and would have a yearly charge split between the other homes on the estate, therefore i didnt' find it out of the ordinery. ( I have had these private bills)

When my partner moved in with me more full time he questioned the water sistuation and we signed up to anglian water online in 2020. It gave us a direct debit option, of which seemed very high but we set it up and my partner pays this from his bank monthly, and has never missed one of these payments.

When the default appeared we rang Anglian water imediatley, the lady on the phone explained that the letters with the incorrect name and post code where 'Just miss spelt', and any evidence that we have to show my partner was not registered here when I first moved in to be sent via email. We have sent across when he signed onto the electoral roll, a tax return dated for 2019 for his old address, the mortgage stating only my name, and a further utilty bill for my address, stating my name and the correct post code. We also sent an image of one of the miss spelt anglian water letters.

We are yet to have a response from Anglian water. We have also been in contact with experien who have contacted Anglian water, and Anglian Water have responded to experence stating the default is correct and is not to be removed. I do not belive this is a response to the email we personally sent, although we are getting very concerend that this defualt will not be removed, and he will not be able to purchase his house.

Sorry this is so long winded, and I hope it makes sence.
Thank you x