No worries Andy,
Hopefully, you get it sorted.
Although I haven't put it to use yet, the issues you describe is why I decided to go down the Baader MPCC route. Not the cheapest option but, as I understand it, there is no change in back focus distance with the MPCC.
I managed to pick one up secondhand for considerably less than new price.
http://https://www.baader-planetariu...-newtons).html