Cannock is a very narrow place sometimes, on both monkey and dog. I can manage to get my 740mm through without much problems though.
Rock gardens are all about speed and confidence. You need to be at a good speed and follow through without indecision. If you panic part way through and try to turn or slow down then you're just asking for an off. The red rock gardens at Cannock are fairly narrow without much line choice so just follow the obvious bit and go for it, try not over think it. Your big 26,27.5,29 wheels wont get stuck on them tiny rocks.