Been to Vauxhall and they want £19 a blade. :roflmao:
Halfords don't do the combination required ie 21" and 18"

Not been to VW to see if they have an aero 18" blade.
 
The passenger one will need trimming down still, unless you angle the arm up a bit.
Go to halfords and ask to try a 22" / 18" combination and report back ;)