I gotta say it as I find it here I'm afraid.
As much as I like Gardiner's products and even the Gooseneck valve, I don't recommend the Gooseneck valve on the Xtreme poles. Whereas it works well on the stronger SLX and CLX poles, it requires lateral force on the pole in order to operate it otherwise the brush will just flip over on the glass. This is no issue for the Xtreme poles when using them collapsed or partially extended, but when fully extended or near fully extended I believe it is a big issue. I broke a perfectly good 2 year old SLX18 in December when my Gooseneck valve stuck and without thinking I applied a little more pressure to the pole to keep the brush down whilst I tried to open it.
The Univalve is a far better option on the Xtremes in my opinion as it requires no lateral force whatsoever on the pole to operate.