Ha ha, I believe you....honest.
Deffo sounds like there's something wrong there. Obviously it's normal to see the MPG drop when stood still (although to be fair the start / stop should mitigate a lot of the losses providing you are not in the middle of a regen). When stood still mine obviously drops, but not like a stop watch like you say.
I can crack 50mpg pretty much every night on the way home, even with the cold temperatures. Also, it's important to understand that where I work is 100 yards from a roundabout, and that roundabout leads into another 200 yards further on. The whole of the business park and surrounding area where I work funnels through these 2 roundabouts as they are the only access to the nearby M62 motorway, and it's gridlock every night. What this means is that no matter if I leave work at 4.30 or 6.30 pm, that first quarter mile takes me anywhere from 10 mins to 30 mins to complete, and I can still crack 50mpg on my 15 mile journey home cruising at no more than 70-72 mph (can't really go any faster anyway). If I cruise at 80 (if conditions permit), this will drop to about 45-47 mpg (pretty much the same as MH said).
You may have seen the thread recently with I took 4 large adults down to Cardiff (with a fully laden boot) for a weekend, and still managed a respectable 56mpg for the whole weekend. With just the wife and I and no luggage, I think I might even have cracked 60mpg !