Is there some trick to get them to fit ? I got the 20" and 21" but there is about 2" of overlap on the passenger side on the 20" ? (On my LHD van) (Im presuming the shorter one has to go on the passenger side)
I have fitted countless 21" and 20" wiper blades and never, ever had any trouble.
LHD, RHD early, late, whatever..
I take it you have the right wiper arms?
I still (in the spirit of paint your wagon for £50) get my blades from pound shops. I buy five sets at a time and change them fairly regularly. They're cheap enough lets face it. Also experimenting with different sizes is viable then. 18" seem fine.
Some blades are slightly different shape to others. I had 2x20" on my van. Drivers blade got close to the top of the screen so a 21 wouldn't fit, passengers side went slightly over the rubber so was too big.
I've just changed them for a 20" driver and 19" passenger and theres now alot more space so its obveous I could fit a 21/20 combo of that type - compairing the new and old 20" blades the adaptor clip that fits over the hook on the wiper arm locates it slightly further out on the old blade, so som brands clips will mean you need an inch shorter, anoyingly as the old blades wern't branded I have no idea what they were.