I'm very grateful for all the help and advice from the forum. I am happy to send the web file and ask for your suggestions, but having thought about it a great deal in the last week I can see that whatever remedies are suggested, I will need a great deal more time to put them in place. I'm thinking particularly of the fact that the site is obviously too large for its situation and I shall have to do the splitting up that has been described to me. I've seen it in other websites, although I've not tried it, but I can see that it would mean a lot more work and time on my part, which I simply don't have - and after trying to get this done over the last two years, my motivation and energy levels are pretty low! Because most pages take over a minute to load I haven't even yet seen my latest version properly.

The website is nearly finished content-wise (although much cut about to try and get it to work here, and I would want to restore it to its proper size) and is very long overdue for going back up for use. So my options are two.

One is to go back to Dreamweaver - I only left it in order to make the handling of the website easier for my successor, but it did its job perfectly for me for eleven years - I have a copy of it still on local hosting and use it almost daily but only I can see it of course: I no longer have Dreamweaver installed (lost in a crash). But I have to rule this out because I'd need to catch up with the latest version etc and remind myself of it, and I haven't got that sort of time now.

Second is the professional option, which I shall have reluctantly to do. I've never dealt with a firm before, but that's a different problem, not for here.

As I said before, I'm extremely grateful for your responses, and that there is such a thing as a forum with people willing to help freely. (I hope that I've already made it clear that all I do connected with websites is entirely voluntary and non-commercial and I don't profit financially from anyone else's ideas!)
Now I shall start the process of finding the place to hand it to, something I've never had to do before, but it will be a weight off my mind. I ought to have stuck with Dreamweaver and let my successors have the worry!