Interesting, I worked for PYMCA way back before the dates you mention, I know the owner sold some images to Rex, which was bought by Shutterstock Editorial, but I’m not surprised to hear about your experience, unfortunately it happens allot.
I would say you don’t need a lawyer, simply demand a list of the images they hold, the sales they have made to which magazines and media outlets, and a list of dates, and most importantly the fees they charged.
Once you have this, you can get to work. Review your rights via the government copyright website.
Good luck
malcolm