Landranger maps (1:50,000) are great for road riding but I'd recommend 1:25,000 maps for mountain biking. It's difficult to see all the detail you'd want for serious off-roading at Landranger scale.
I managed to get lost in the Highlands using a Landranger map for a spot of MTB exploration. Carrying and pushing a mountain bike for 5 hours across a peat bog wasn't much fun

Mind you, I wouldn't have thought that there are many huge peat bogs near Glastonbury...