Spot on. Apart from postage and packing, 17% in absolute terms is all I've ever paid on top of bid price.
With regard to the eventual bid price compared to what I actually bid via e mail, these are the details from my last 5 successful bids.
March 2013 auction, lot No 2112, e mail bid £375, actual winning bid £360.
September 2013, lot No 2655, e mail bid £650, actual winning bid identical, £650.
December 2013 auction, lot No 2166, e mail bid £451, actual winning bid £420.
June 2014 auction, lot No 1923, email bid £200, actual winning bid £180.
September 2014 auction lot No 2732, e mail bid £100, actual winning bid £80.