Thanks for the suggestion! I will do that, as I do have a Dremel and cutting disc. I think that space may become larger as I think I need to move the battery further back. I moved it back by about 1.5cm from what the picture shows, and noticed an improvement in how it flies. (Even though I'd checked the C of G and thought it was ok).
2% Thrust. I returned the rudder to 0 trim, and I noticed that as I flew verticle, approaching a stall turn, there was a significant difference, no veering to the left! So I can't help but think that a small percentage of thrust may help.