you should point out point 47 of the following document
http://www.ico.gov.uk/~/media/documents ... 20DOC.ashx which clearly gives guidance on this matter.
Have you complained to the Financial Ombudsman too? Your IP will probably not want to get involved as this is not their responsibility. It's down to us to resolve these issues.
First of all I'd write to the CRA's to dispute the default, it should not be there and I'd suggest you ask that they suppress this data from your file until such time the creditor can validate the default.
Given that this creditor was included in the IVA, the ICO guidelines are very clear, they should back date any default to the date of your IVA when requested to do so by yourself. They are in breach of their duties to ensure data is accurate and kept up to date based on the Data Protection Act.
The delay from the ICO is unfortunate, I trust you have sent them all the documentation, dates and information needed for them to make a ruling. It does seem they are taking an age to get back but sure they will at some point in time.
The financial ombudsman is another route, as I've said previously they seem to have given you a final answer and it has been longer than 8 weeks so could go to them stating their legal obligation under the act enclosing a copy of the default guidance note above highlighting point 47.
We're willing to wipe the slate clean, give you a fresh start. All that we're asking in return is your cooperation.