Mark can you guarantee that no one has ever put supermarket fuel in? Didn't think so.
Also, given that your company cars all seem to be second hand and heavy on suspension, I would politely suggest that either your Vectras were poorly maintained using shit parts, or you simply can't drive.
As for being underpowered, the 1.8 in both 120 and 140 guises is keen enough and can be driven at the UK limits with no fuss or drama, even fully loaded. If you want to tow a twin axle caravan at 100mph, it would be only fair to suggest that it might struggle.
Even badly driven around town, the estate will return 28-30, and on a run 40 shouldn't be unrealistic.
I know of noone who hasn't had problems with either the 2.2 petrol or the 1.9 diesels... The 120 diesel is fractionally more reliable, but is gutless compared to the 150, but it is also les complicated.
That is my experience based on several hundred thousand miles, take it as you find it