Hi Rick and all,
my problem was indeed caused by differential wing incidence between the two halves of wings. When I checked with incidence meter, I found there was a differential of about 1 degree. Righting this solved my problem and I now have 0 trim on my aileron upright and inverted. I thought I share this with you all and once again thanks for all the input. I am sure they will come in useful in other situations I am sure I will encounter!