Update: I cancelled the E53 and have opted for the E63S.... Hopefully collecting it on Friday, if I can find some diesel for my Audi before then as it's a 150 trip to the dealer!
Mercedes said the E53 had gone into production at long last on Monday, and then when I spoke to them on Tuesday they said, sorry, actually it hadn't, but the good news was the 'Export HOLD' notice on the order which had appeared on Monday had been removed. Delivery date had slipped to November and they still weren't certain what items would be included / excluded (there were discrepancies between the Oasis printout and the 'Change of specification' notice), so I finally decided enough was enough and took the plunge with the E63S.
Next order of business is to order a boot protector to try to make it hound-proof, and some rear set protectors to do the same for the nippers.... If anyone has any recommendations on either front, I'm all ears! I plan to get a PPF applied too, and possibly have it detailed.