Hi Jules, it sounds like I'm in the same position as you, I declined a company car and opted to stay with my Alfa. The only down side is that I'm currently doing in excess of 500 miles per week
I too keep an close eye on MPG and have found it OK if you anticipate traffic so no sitting on the guy's boot lid; smoooooooth is the way to go - and that doesn't mean you have to drive slowly
Rob