Congratulations on getting the certificate, it's an amazing feeling getting hold of that piece of paper.
There is a template letter, that will help you sort your credit files out on the site here:
http://iva.co.uk/forum/topic.asp?TOPIC_ID=31963
Of my creditors Max Recovery were the quickest to respond, so hopefully most of your debts were sold onto them. MBNA took 8 weeks but gave in in the end and did what they were supposed to do.
First Direct were a bit of a nightmare, equifax and call credit were updated fine. Experian, kept putting disputes on my file, associated with my current bank when I was not in dispute with them but first direct and did not request the dispute notice in the first place. The default dates kept going backwards and forwards no less than 9 times until it was sorted in the end.
But to answer your question they have 28 days from receiving the letter to act and update your files. If they fail to do so, you then need to follow up with a more strongly worded letter insisting they do so within the next 28 days or you'll make a formal complaint with the Financial Ombudsman / Information Commissioner.
At which point 8 weeks will have passed and you can take your case to the Financial Ombudsman. You can also write to the Information Commissioner, there is a complaint form on the ICO website.
We're willing to wipe the slate clean, give you a fresh start. All that we're asking in return is your cooperation.