How bad is a credit record default for a mortgage application?

O2 hit me for £10 owed at the end of a contract. Now I worry if I’ll be able to buy a houseQ My partner and I want to buy our first home. We started the process of looking into this last year, thinking that it would be relatively straightforward once we had our deposit saved.We both have good jobs and assumed we had no debt. I then did a check of my credit file and realised O2 had slapped a default on it for £10 owed at the end of a contract. I have disputed this endlessly but