The defaults and IVA's will stay on your credit file for 6 years from when they were first registered. So these should drop off In November 2012.
Until such time you may find it difficult getting any credit from the main banks. Although as Jan says some have been lucky in this respect. Personally I'd leave it until your credit file is back to normal and then apply to your bank.
You can insist that the credit card company update your credit file to show that no money is due, after you get your certificate of completion as the information is no longer factually correct. They have a obligation to do this under the Data Protection Act.
You can also insist that any defaults registered after the date your IVA was agreed are backdated to the date your IVA came into effect.
In terms of the best person to write to for this, you need to find out who the Data Controller is for the creditors with incorrect information at the Credit Reference Agency and write to them and not Customer Services who probably have not got a clue about their obligations under the Data Protection Act.
To find out who the data controllers are, and their address you can do a search on the Information Commissioners website
www.ico.gov.uk
We're willing to wipe the slate clean, give you a fresh start. All that we're asking in return is your cooperation.