Hi Simon,
I have a 10 Reg, GS450h with 31,000 on the clock.
I normally get 28/29 mpg when driving locally. I recently filled up with Super Unleaded however,and got 35mpg when driving locally. This was a Brim to Brim (not OBC) measurement.
I have just returned from a trip to Edinburgh having filled up with ordinary unleaded and got 33mpg, with the return trip giving 34mpg. These again were real (Brim to Brim) measurements and speed was typical motorway speeds WITH NOT MUCH PASSING ME!
I expect to get between 400 and 480 miles from a full tank of fuel and I endorse all that Carl has said about power and performance. The drive is majestreal.
Enjoy whatever you decide to buy.
Regards
John