Cracked it!!! :oD Thanks so much for your help on this Tarpontech, the pointer towards content_extensions.php was just what I needed!
Here's what I did:
1. Read through 'content_extensions.php' until I found the '//creates the content' section on line 476. This section specifies how posts should be displayed depending on the page being shown, it can be either 'full' or 'excerpt'. I changed all the page types to 'full' as follows:
//creates the content
function thematic_content() {
if (is_home() || is_front_page()) {
$content = 'full';
} elseif (is_single()) {
$content = 'full';
} elseif (is_tag()) {
$content = 'full';
} elseif (is_search()) {
$content = 'full';
} elseif (is_category()) {
$content = 'full';
} elseif (is_author()) {
$content = 'full';
} elseif (is_archive()) {
$content = 'full';
}
I know I could leave some of them as excerpt but I went for the blanket approach to test the theory.
Now all of my category and archive pages show the post and a 'Read More >' link.
I also have a 'Page of Posts' page which is based on a custom template that I found. The more tag doesn't work on it by default but can be enabled by adding the following code in before <?php the_content() ?>
<?php $more = false; ?>
Thanks again Tarpontech, couldn't have done it without your help!
Ollie