OK I managed to figure this out on my own and done it without hacking core Virtuemart.
Managed to do it as an overide & would just thought I would share.
I copied the file in com_virtuemart/views/productsdetails/tmpl/default_reviews.php and moved it to templates/MYTEMPLATE/html/com_virtuemart/productdetails
Then in line 79 I Changed this line;
<span class="bold"><?php echo $review->customer ?></span>
to
<span class="bold"><?php echo $review->username ?></span>
Feeling all chuffed with myself now! :D