Restart your computer, and do not try to launch Lightroom.
If the lock file is present now, go ahead and delete it, then try to launch Lightroom.
If that's not the problem, you have a permissions issue, make sure to have READ and WRITE permissions to the folder containing the catalog.
Thanks for that. A delayed response as Lightroom opened at the seventh time of trying and then I've been out for the day. However, I will follow your advice the next time I restart the PC and see what happens. Cheers.
Well, I re-started, deleted the ircat.lock file and Lightroom opened straightaway. Let's hope it continues to do that! Thanks for the advice.
Just so you know, that lock file is normally deleted when Lightroom closes if it closes normally. If something happens during the shutdown process to prevent a normal shutdown that lock file can be left behind.