Thanks for the reply Shad, very informative and a million times clearer than what anyone else hase told me!
The place I bought the dizzy from (an aircooled.net supplier to the UK) told me the vacuum retarded the advance!
I'm not entirely sure what aircooled.net's description of how to set this thing up was telling me but I understood it to be that basically it gave you an extra 10 degrees or so max advance when cruising.
Both of these seemed 'wrong' to me.
It may be that I have missunderstood what the suppliers of these parts have been telling me or it may be that they just plain got it wrong.
At the end of the day I suppose that Mallory should provide a clear description of how the Unit works along with at least some idea of how to set it up.
Thanks again for your time.
Rich