The new armature has fixed mine, Mark and I went for a quick spin today and all is fine, I went to work on it - and at first I thought it lacked the performance, but when I swapped the battery for the Sinclair one - it was fine (I think my cheepo 38 is on its way out). I am so glad I saved the Gearbox - Aralditing the gearbox pin in place worked a treat. Gave it & the pins and the bearings a good greasing (this time putting only enough tension on the belt required to stop beltslip. I slackened off the the other c5's belt too (once bitten).
I hate to speak ill of the C5 - but its not the most powerful thing in the world, and might very well be as it should be. The only real way is to try another or better still take it and another out for a ride at the same time.