I spoke to the dealer a few days ago and queried it. Apparently, all the cars in the south are 'staged' at a service depot in Weymouth where the PDI is done and any extras fitted (auto wing mirrors in my case). He suggested that cars are then driven from there to the dealers. However, Weymouth to Slough (in my case) is 120 miles so clearly that didn't happen!
I doubt I will ever get the truth - but talk about starting off on a bad foot with a dealership...