All's well that ends well..
Turned out versions 1.58 and 1.205 are compatible (unlike previous versions) so all I needed to do was update the version in the RDB.
So I got all annoyed for no reason. Don't I feel silly...
BTW, I recommend SFS to everyone. It's halved my boot up time, and IBrowse caching seems to fly now.
I haven't crashed the system since installation of SFS so I can't comment on the stabilty yet. But from everyone's comments it sounds like I'll never have to wait for validation again.
Thanks to everyone for their advice and comments.