Rebuild Your Credit
If, after checking your credit rating with the credit reference agencies, you have confirmed that your credit rating is indeed bad, there are several things you can do to try and improve matters.
Check carefully that all the details on your credit record are correct
It is not unknown for your credit record to be out of date. You may have defaults listed that you have since cleared - for some reason it often seems easier for a bad debt to find its way onto your credit record than a cleared one!
Clean your credit file
In some cases it's possible to remove CCJs as well, although for this to happen you'll have to prove that the CCJ was issued in error, and not that you have simply cleared the debt since the judgement was issued.
Check that all the details actually apply to you
In some cases, the debts of a former spouse, previous tenant/owner of your property, or other family member can appear on your credit record. If this happens to you, you can ask for a Notice of Disassociation form from the credit reference agencies, which you can fill in to legally distance yourself from the debts of others.
Pay off small debts
Try and settle as many of the small bad debts as you can afford, and ensure that this is reflected on your credit report.
Apply for credit
This is not as strange as it sounds! By applying for credit with a company that caters for those with bad credit, and is likely to accept your application, your credit rating will be strengthened. Successfully obtaining finance, such as a credit card, and responsibly maintaining your monthly payments will help offset any 'black marks' on your credit record that you've picked up in the past.