Michael, thanks for replying. As it happens, while you were posting I was nosing around in the Lunarpages Wiki, found the pages on PHP and learned about creating php.ini from scratch, with the associated mods to .htaccess.
I put a 64M limit setting into a php.ini file which I copied into my public_html directory, also inserted the required additional lines to .htaccess, and that seems to have done the trick.
Thanks again to you, and also to MrPhil, for taking the time to help. Much appreciated!