I'm a newbie here

When I was a student my credit wasn't great, had various loans which I really struggled with but have been working hard to improve my credit record over the years.
A few days ago I applied for a loan from Sainsburys Bank, having had one which I cleared off around 18 months ago (no payments missed) so thought it wouldnt be a problem getting another in order to upgrade my car. I did an online joint application with my Wife (who has a perfect credit record) and it came up "approved in principal" and that the credit agreement would be sent in the post. The following day I received an email saying that I had to call Sainsburys in 72 working hours to finalise my application? I did this tonight and to my dismay was told my application declined! I checked my credit file online and the results were as follows:
Credit Scores & Ratings
Using data seen by Lenders
Callcredit590 Equifax563
Using data seen by Landlords/Employers
Callcredit819 Equifax819 Experian757
Other Scores
Postcode909 Application796
I have never missed a mortgage payment or any loans, etc with the exception of 1 black mark on my account. It was an overdraft with a default date of 2010 (even though this must be around 8 years ago at least when I had this!!). It was a result of interest being added and me not being able to afford to pay the overdraft and interest at the same time so a £500 overdraft ended up being £3000!! It is with a debt agency now, has been for few years and I will be honest in saying I have ignored it until now. It says it will be reported on until 2016.
Is this 1 default really causing me all these problems considering everything else has been paid on time (and I even pay more each month to my mortgage than the minimum payment)?
Any advice would be appreciated. Thanks