Thanks to Robert for resolving my earlier feckwittedness - http://forums.invisionpower.com/topic/292886-import-users-from-phpbb3/page__pid__1855635__st__0&#entry1855635

I'd started the conversion process and seems to be going OKish until I try to convert the members (7774 of them), it seems to start the process then whitescreens on me - are there any other settings that need to be addressed?

Cheers,

Just tried to increase the number of cycles to do 100 members at a time and at least I've managed to move forward. Thought I'd leave this up as a reference point for anyone else encountering similar issues.