In reply to:
Which page gave the error you mentioned? The main page or somewhere down in the site?
I can't remember. I picked one at random, rather than checking them all. After all, that's your job...

. I'm sure the validator will find it for you.
For converting spreadsheet tables to HTML, that's something I don't have experience of doing this, but one thing that might work is to use your spreadsheet's export facility and run it through Dave Raggett's
HTML Tidy, which I've heard good things about. Alternatively, export as something simple like CSV and write some Perl to generate the structure.
In reply to:
The best part is that we've gained about 10 new members because they found the webpage
Yeah, I've had that experience several times over with the local
student hillwalking club. It gives me warm-and-fuzzies to know that I'm helping people have more fun.
(And yes, I have to confess that I don't visit the link checker as often as I should...)