Will a 5 year old default prevent me getting a current account?

Is a default from 5 years ago enough to prevent me getting a current account, or is there something else to it?

5 years ago I was in a dispute with my mobile operator 3. I just stumbled upon my correspondence with them and I clearly had a case to terminate my contract with them. Is it too late to revisit the case to try and get their default removed from my account?

I was unable to text mobile numbers beginning 078... (I called on day 13 to cancel, an engineer convinced me the problem would be fixed, when I called back on day 14 they said I was out of my cooling off period).

I stopped paying my bill and using my phone whilst the dispute was ongoing and they put a default on my credit file which I paid and settled as 3 refused to back down. I didn't know about credit files/scoring back then.

I think it's haunting me now, I have no missed payments, a stable job with a £30k+ salary. I was refused an emergency overdraft when I was on holiday in Croatia and my job missed me off payroll. Worse still, when I tried to change banks in protest I was declined a current account at another bank because despite my "credit score being acceptable" there was some "other information" on my credit file that worried them.

I'm worried this could affect a mortgage application should I wish to buy in the next year.