My experiment at this stage is quite raw. I simply removed the body and placed 6 coins on top of the power car. The results are quite pleasing. Clearly I need a better weighting solution but this definitely confirms lack of weight is the issue preventing slow running and causing severe wheel spin. I do have running videos on my Chesborough Junction & Healey Bridge Facebook group.
I now have 2 and out of the box they run awful and suffer severe wheel spin, particularly when going over points. Today I carried out an experiment and removed the body and added weight to the power car. This seems to improve running considerably.