I've just moved the CofG back quite a way on my 3D foamy as it required a LOT of down elevator in inverted flight compared to my other planes and suddenly since then where knife edge used to require no elevator and was "sweet" to fly around the field it now pulls quite heavily towards the undercarriage in knife edge and needs up elevator to compensate.
Is this normal / can anybody explain to me why that should happen ?

Split the difference then mix some elev, w/rudd. Which way did you like best. If giving elev. input inverted is not to bad for you and everything else works good that way then well the choice is yours.

Not a very good answer i know but i don't know alot either. Did not know changing the cg would effect a KE that much. We all learn on here i guess.

When you move the CG back, now in level flight you will need less up elevator. SO when you roll knife edge, there is a tendency to pitch to the belly.

Kid Chuckles,
Must admit I was suprised how much effect it had, it was lovely to do knife edge with before and so easy you could just fly otright the way down the field then just flick it on the other side and fly ot back again, granted I'll get used to it but now I really have to concentrate but inverted is much nicer and also am learing rolling harriers and you ha to give it so much down it wasn't true which is now much better !!!!
Swings and roundabouts really !!!!

Great glad your liking it. I have a couple planes tha KE great and then a couple that pitch to the feet. (landing gear) I mixed some elev. and rudd on one of the pitching planes and had not a clue as to what percent to go by but quickly learned that it does not take much. I guess on my Frenzy 100 size i have about 5% mix started with 15 and man that was way to much. First time i tried mixing and was a little nervous as to how it would fly regular when i used the elev. or rudd. But it if fine with just 5%. I have an addiction and an Eratix that KE great without having to mix.
Again glad your liking it and very good luck to you. 3D is an Addiction in itself lol.