No straightforward explanation in the manual supplied with the upgraded version of the module....

Could you perhaps explain why we are making the changes you advise to the template file

(<?php echo "".$_SERVER['HTTP_HOST'] ?>)?

Is this because of the way PayPoint opens the success page in their own URL ? If so, does that mean I can do away with my earlier changes, requiring me to reference my stylesheets (in index.php of my template) with an absolute URL ?

Many thanks

Yes the confirmation (callback page) is opened with PayPoint therefore URL's need to be to a fixed address.
Which I know is pain! Becuase it means you need to change your index.php template file. This has to be done though because otherwise the CSS files won't open and the site will have no design for just the success page. This is how PayPoint operate. We have discussed alternatives with them, which are currently being considered.

Using <?php echo "".$_SERVER['HTTP_HOST'] ?> is just one of the many methods that conquer this issue.

You can also link to your CSS like:
www.mysite.co.uk/templates/theme-name/css/stylesheet.css

Therefore in theory if you've got an absolute URL links at the moment to your stylesheets then that should work fine.

I would prefer not to use the absolute links method as I currently am so I think I'll make the change you suggested.