Just to update this question; I have just trashed the preferences files (following instructions on this site) and have still got the same problem.
I've tried 'Threshold', 'Light rays' filters which both crashed and then the Gamma correction which worked ?!
If it's of any help the extensive crash report includes this near the top:
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000050da6bee
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
well Preference Manager eliminates the possibility of operator error. It's free and it also allows you to back up your preferences.
Digital Rebellion also has as a crash analyzer which is part of one of their suites of media utiltiies (Can't remember which one, but go to their site and do a bit of digging. You'll find it pretty easily). Although it's not free, it's a worthwhile purchase and there is a free trial available.
The mac os has permissions for each and every file and the os uses these to determine whether the user is allowed to read, or change a file. They can get corrupted but I've rarely been able to solve any problem by repairing permisions. but as far as I know it can't hurt. Just go to applications: utilities: disk utiltiy and repair permissions on your startup drive.
Have you installed any new software or done any software updates recently?
I've repaired the permissions via disc utility but no joy.
I also went through the basic FCP troubleshooting tips (excellent list btw) and followed many of the suggestions.
The only one I haven't yet tried is the prime one of running disc utility via another start-up disc. I tried booting up from Disc Warrior and it just sat there with the mac start-up logo for ages and ages. Sadly I can't find the original imac install disc to try to boot up from (yet).
I'm beginning to wonder if I should un-install FCP studio (is this fiendishly complicated?) and then re-install.
Obviously my fear is that after a lot of hassle the exact same problem will still be there.
I haven't installed any new software and I haven't done any system updates for ages. I'm one of those people who gets a system nicely balanced, working and reliable and then doesn't fiddle with it. It could be that I DO need to give the system an update of course, but that could simply add another whole load of complications.
Basically, it was all working fine, then it wasn't.
Thanks for your help and suggestions
Did you download digital rebellion's pro maintenance tools?
There's a free trial and you can use crash analyzer to get some insight into what's going on.
If you clone your startup drive, you'll be able to boot off the clone and run disk repair and diskwarrior off the clone.
I think carbon copy cloner is no longer free. If so, check out superduper
And once we solve the problem, and get things back to a working state, you should definitely clone your startup drive. That way, if things go south (like they have now), you can clone back to your internal drive and get back to work.