Help with Walkera Master CP after crash

Had a crash which took out one of the servos so replaced them all with metal gear ones.

Having problems getting it set back up. I centred servos and set the servo arms so they were in the right place for the swash plate to be pretty much level but when I fire up the controller the "neutral" position moves the one servo down (but a significant amount) so the swash plate is no longer level annd that servo will be much closer to the one limit. I have tried binding as a fresh model to see if that is the issue but does not make a difference. Any suggestions? Would adjusting the screws on the receiver helpwith this?

Perhaps you should try the following.
Remove the linkage from the servos to the swashplate. Swap two servo connectors on the receiver and observe whether this behavior remains with one servo or whether the other servo behaves in the same way. This could help identify the cause.

The issue isn't necessarily the servos themselves, but the differences between the old and new servos, even if they're the same spec. Metal gear servos *can* have slightly different internal centering points or travel ranges than plastic gear servos. Your controller is trying to achieve the same "neutral" position it had before, but the new servo is interpreting that signal differently.