hmm i used to have bad credit back when i was young when i got my first credit card thinking the overdraft was money i 'owned'.. boy was i in for a treat. To make things worse, i had a dispute with O2 as they mistakenly charged me for something i did not owe which ended in me having a Default on my credit rating.
The way i went about this was to first clear as much debt as possible, even just paying a bit more than the minimum amount per month to get back to 0.
Might be a bit counter-intuitive but every couple months or so, i would apply for a credit card... (that is key as everytime you are searched.. whether by Experian or what not, can impact your rating) regardless of who it was by and what interest rate it had on it. I would then make sure to spend no more than a tenner on each card and set up a direct debit/standing order to pay off the balace in full each month. This was a good way of recovering your score as it shows you are using your card often but also paying it back in time... the only down side is, it's bit of a double edged sword as having too many credit cards can hurt your rating too, and if you don't use the card at all, at least close the account otherwise that too can have an effect.
It's took me roughly 5 years to get my rating back up to where i consider myself financially stable and was able to get a decent mortgage for my first house