Finally found one that works - Genuine is definitely better.
Having replaced the original 10yr old washer with a non-BMW replacement after 6 months the replacement began to fail. I then tried a reconditioned replacement. After a month this washer also failed. I decided to source a genuine new replacement.
This has now been in place for a couple of months and is showing no signs of failure. The quality and effectiveness are a big improvement on my previous attempts.