I have serviced loads of Mk3 MX5's and do not believe that the engine is canted 10 degrees to the right!
Can you link to the source of this info Dave.
Although I found this yesterday, today I can't find it. I'll look again though.
Not sure if it's just later models, but it said the engine is canted 10 degrees to the right to make space for the induction system.
Ten degrees isn't much, but the level plug on Plus 4's is visibly low.
As an aside the Duratec in Plus 4s isn't centred either, it's offset, but unless you look hard, you wouldn't know!