Well I think I ahve a fix.. but I don't know why yet.
Changing the line to : "$catimage = get_thumbtmpl( $id, $thumbrc, $height, $width, $checklink);" (removing the last variable passed, $pcat) makes everything work again properly.
Ah well there we go... $pcat is say 16, which is the category ID tag, get_thumbtmpl then correctly operates and goes to this loop:
if ( $Globals['spider'] == "yes" )
$murl = "<a href=\"{$Globals['maindir']}/showcat.php/cat/$whichcat/ppuser/$theuserid\">";
else
$murl = "<a href=\"{$Globals['maindir']}/showcat.php?cat=$whichcat&ppuser=$theuserid\">";
This leaves us with the incorrect "showcat.php/ppuser" links. get_thumbtmpl is operating properly.. well as it's written, however it's outputting category style links instead of product links because showcat.php has sent the wrong variables across.