Ive installed the replacement mainspring and it’s running on its own. Problem is the amplitude is quite low. I double checked my measurements on the old mainspring and am getting thickness values from 0.15mm (what I replaced it with) to 0.18mm. I wasn’t getting this level of inconsistency before. It makes sense that if I’ve undersized the mainspring it would have this result. A 0.18 thick mainspring will be 1.75 times stronger than a 0.15mm all else being equal. New ones are on the way (0.17mm & 0.18mm). It runs the same regardless of the position I hold the movement in which is also fantastic news.