Seems likely that your file is damaged in some way. Try performing a recover on it and see if any problems are reported and check the stability of the recovered copy even if no problems are reported.
Best practice is to never use a recovered file going forward, but to replace damaged files with undamaged backups if at all possible.
Thanks for you help PhilModjunk - have performed a recovery, and it does state that recovered file is unsafe to use, so I renamed this recovered file as file 2 and performed another recovery - this time statement said recovered file safe to use.
Does this mean that I can proceed safely with this altered file? (which is a copy of the original damaged one).
Not if you have any reasonable alternative. Otherwise you are relying on Recover to have fully and correctly fixed the problems with the file. Maybe it did and maybe it didn't. Thus, it's safer to replace the file.
If you don't have a good back up copy, you may need rebuild the file from scratch.
Rebuilding from scratch is not a good option for me - I have recently finished manually inputing 1400 case records from my old computer and filemaker pro 2 file. I regard my updating of computer and case records to be one large and now increased nightmare!
To go back to the beginning and start again would involve another 4 months or more of "after hours work"!
Will this problem eventually be solved by downloading updates of the filemaker pro 11? - have downloaded one a few weeks ago, but this has made little difference.
It's unlikely that any update will fix damage to your file.
We need to keep separate the data and your database design. I'm suggesting you rebuild the fields, scripts, layouts etc of your damaged FileMaker file--not re-importing data from an earlier system. You can rebuild your database file and then import the data from your existing damaged file. Importing 1400 records from your existing file likely will take just a few minutes to import into your rebuilt file and can be setup to run overnight or at another time when you don't need immediate access to your computer.
There are a couple of third part tools you may want to investigate: FMMIgrator which is supposed to help move design elements from one file into another. and FMDiff--which can give you a much more detailed look at any possible damage that may exist in your file.
You can also import scripts and table definitions from one file to another and you can copy and paste layout elements from one file to another, you'd need to carefully check your new file with recover runs or an FMDiff scan after each such move to make sure you didn't pull in database corruption with it--but it's still much faster than typing in field definitions and recreating scripts one small element at a time.
Even with all of the above, rebuilding your file ill still be a lot of work--just a bit less than doing it one tiny element at a time.