just an update on this prob.
Bought the new tube from nissan...and found that it is slightly different to the original.
The old dipstick doesn't fit it!
I went back to the dealers and they said they didn't know why this should be the case as it is the only part for the car. They checked for me and it turns out the design has now been modified and a new dipstick must be bought at the same time (there was no notice by nissan to the dealers). So it turns out it must have been a design fault with the dipstick after all......maybe they should have recalled them earlier??
