Thanks Adam; I put a custom function into the loop and tried out various adaptations of your code above in my theme's functions.php file, but just couldn't get it to work (I'm a PHP novice).
Adapting this fix I tried the following:
<?php _e(urlencode(the_title_attribute('echo=0'))); ?>
It works (that is it satisfies "Check syntax" in BBEdit and Tidy; it's worth noting that the malformed URI fools the W3C validator - which is probably why nobody much cares!).
However, this encodes the spaces with "+". For example the title of this post, when I click "Send" (Actions box), becomes:
I'd rather see:
From Proto.aculo.us to jQuery
Any further suggestions?
BTW the functions are defined in wp-includes/post-template.php in WP 2.3.2