Hi Phil
In the unlikely event of an insolvency practitioner going out of business, with no successor firm lined up to take over the practice, then the IP's own regulatory body would step in initially to deal with those cases, and offer them to other insolvency practitioners to take over.
I have actually been involved on the other side of this twice in my career, and the IVA cases I have taken over have run quite successfully to conclusion with little disruption as a result.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk