I found a very useful link on the http://www.bankruptcyhelp.org.uk/ website posted by a user called DebtInfo (thank you my friend). The link is to the official figures for IVAs, including all failures (it's in spreadsheet format). Basically it shows that the typical failure rate for IVAs runs at about 30%. That's almost 1 in 3.
With my situation I discounted IVAs relatively easily. These figures should be examined by anyone wanting to take up an IVA - I'm not against IVAs, but I think people should know what they are getting in to.