Thanks for the input Acorn. I decided that in some way the whole xar file was creating too many issues so I totally rebuilt it anew. Often far better than chasing down Xara's sometimes buggy output.
Works fine now even in FF with no horizonal bars or shifting. The PageSpeed Insights are now sorted and everythings in the green.
Not finished but bug free now :D
https://parkeston.com/hc20/
EDIT I don't understand why the pasteboard background should be such an issue. The pasteboard is just repeat filled with the bitmap below ???