Firstly, if it's a newish lex, it will start to get better after the first 3500 miles.
To get more out of your tank, try this.................
When crusing at 70 (I know, seems VERY slow) feather the accelerator and let the speed gradually slow down to 65, then speed up to 75, and repeat.
When setting off from roundabouts, accelerate hard to get to the speed you want to be at and then feather acclerator again.
This was, however, 1/4 unleaded, 3/4 Optimax
I think the three key things are:
1. get to the speed you want to be at ASAP. Don't go through the gears.
2. Engine brake: AFAIK, when coasting in gear with no feet on the peddles, so long as the revs are above a safe RPM, the ECU cuts the fuel
3. Do the speed up, slow down naturally thing. I think that's the key thing.
For the first 1/2 a tank, sitting at 70 produced just less than 40mpg on the gauge. After that, it was slightly more, probably due to the lesser weight of fuel being carried.