I had no idea when I started this thread that it would be so difficult to explain what I saw as my problem, but your questions have made me re-examine my assumptions.
This link will open the 'Terms and conditions' page of a B&B web site. http://www.mull-bed-and-breakfast.co.uk/terms. .htaccess creates a query string, index.php parses that and attaches the correct argument to the PHP 'require' statement.
So far so good, but what happens when I want to direct the visitor to the paragraph at the bottom of that page about dogs (for example) ? I made the assumption that I couldn't just add the bookmark as in http://www.mull-bed-and-breakfast.co.uk/terms#dogs. Just to be sure I tried it, and it didn't work, thereby confirming my assumption that .htaccess would choke on it.
However having persevered with this issue since my last posting, it looks to me as if the reason it didn't work was a typo. I hope that if you click the second link above it will now take you to the intended place ? If it doesn't I'm back to square one.
It seems to work OK for me, which means my work with the <input> and JS was unnecessary (although that did work too), so I expect I learnt something.
While investigating I've also added rules to .htaccess to prevent pages being loaded outside the template, so something useful has come out of it.
Thank you for your help.