Richie
I've got a 2001 330i manual and regularly get 420+ miles on each tank on long journeys. I took it to the Nurburgring a few months ago and it averaged over 37mpg with the tyres nicely pumped up in preparation for the circuit.
Over the last few weeks I've been doing 20 minute journeys to and from work, in freezing conditions and I'm always running late yet it still doesn't drop below 30mpg over the week. I've checked the accuracy of the fuel computer with miles vs litres and it's almost spot on.
If you're using a lot more fuel than that, check your tyre pressures. It makes a huge difference although if you have them too high it ruins the ride and reduces grip. Also if yours is an auto then it's a different ball game. And being a convertible it will weigh a bit more too. I always use VPower or preferably Tesco 99 fuel. It's probably all in my head but I reckon the car pulls a bit harder, runs more smoothly and uses a bit less of it.