I am awaiting delivery of a Yeti (round about the new year, hopefully) and as part of the deal clinching I accepted a free Supaguard treatment. Is there any benefit - or indeed any danger - in applying polishes such as Autoglym on top of the Supaguard to maintain a showroom shine?

No danger at all, results are great, ranging to the questionable - it all depends on the man (or indeed, woman) applying the system. Basically it's a wax, which isn't actually a wax but a polymer sealant that acts like a wax and lasts a long time. I think they say three years in their bumph.

As above it is largely dependent on the correct application by the dealer.

You will definately need to keep on top of waxing/sealing it along with a good, safe wash routine as the initial dealer application doesn't last very long