To all you 29er's out there!

There's not many in London either... but if you have a mud/gravel/clay canal or river path you don't really want to go so fast that you don't have control enough to stop yourself taking a dip.

Understood. Guess my circumstances differ as I only tend to ride for up to 2 hours and ride for fun/fitness so go as fast as my fitness/skill allows.
64GI is a good offroad gear for that purpose, any lower and I risk getting overtaken by pesky geared MTB riders.