It is up to the Credit reference Agencies to make sure the data they hold is accurate ( Data Protection Act). If they don't arrange to correct the information, you can make a complaint to both the FOS and ICO.
If Lowells have been legally assigned the debt, then they can register their details, but the date of default should be corrected. I don't know what happens. Do they just add their default information and it is up to Barclays to remove their entry ?
In this situation, I think you should be making an official written complaint to Barclays and Lowells, about this issue and about the charges applied to the account. Advise that you intend to involve the FOS, so if they are unable to resolve, within 8 weeks, you will have no alternative, but to take up the complaint with the FOS. Lowells will not be happy about this, as if a complaint is ongoing, they won't be able to chase for the debt and therefore they may pass the account back to Barclays, Barclays will then have to keep the account, until the complaint is resolved.