I have never been totally satisfied with the wipe of the rear wiper as it is used so infrequently that starting from a clean wipe is nigh on impossible and it needs a few wipes before it wipes clean.
I sceptically tried some 18" wipers (20" not available) from the Pound Shop and found they performed as acceptably on this rear window as the regular brands. What's more, they come in a pack of two so for 50p per wiper you can afford to change it more regularly if you wish. The missing 2 inches is also unnoticeable as the top of the wipe cannot be seen in the rear view mirror anyway and the lower bit is beneath the spoiler.
For the front, I will continue to use 20" Valeo but at the moment I can't think of a reason not to use these cheap wipers on the back
