Hi silly billy,
You're right! I just reviewed the eBay Motors(UK) fee structure, http://pages.ebay.co.uk/help/sell/motorfees.html,and it appears the reserve price fee is GBP 3.00 regardless of the reserve price level set.
In this case, it sounds like our "Estimate eBay Fees" calculator is off however, not to worry, you will still be charged the appropriate insertion fees according to eBay's fee structure when your item is posted.
-Mike