Fitted new hanger, no improvement. Trip to Fred Williams bike shop, diagnosis, sticking cable. New outer at the back, job done. In my defense I had checked for this and cleaned that outer by running a spoke through it but under load it was still too tight. Anyway it's fixed now, thanks for all the help and advice, sorry for doubting shimano, it won't happen again!
Fitted new hanger, no improvement. Trip to Fred Williams bike shop, diagnosis, sticking cable. New outer at the back, job done. In my defense I had checked for this and cleaned that outer by running a spoke through it but under load it was still too tight. Anyway it's fixed now, thanks for all the help and advice, sorry for doubting shimano, it won't happen again!