Tim Watts wrote:
I can change the layout generally, and bugger up the inbuilt keyboard's
map - but I cannot for the life of me work out how to use 2 keyboards at
once with the correct maps applied to each.

Have a look at Karabiner Elements. You can modify keyboard events from
specific keyboards. So it will allow them to work differently, but you
might have to manually create mappings for the keys you want.

Theo