Thank you. It seems that almost everyone in this thread is suggesting changing the frame, so that's the advice I'm likely to follow.I'll describe how to put a frame in but only if it's what your doing.
I've watched several videos on YouTube and have a general idea of what installing a new frame involves. The challenge with those videos is that they usually demonstrate how to install a frame into a relatively straight doorway. I would be surprised if my situation is that straightforward.