Thanks for the advice guys!
And I certainly feel a difference when I fill up my car, it seems like it just...goes faster or runs better (not that it runs bad anyway) when its been filled up, as odd as it sounds.
There may be actual technical reasons for this, firstly, the ECU may go into a "fuel saving" (read low power) mode when the low fuel warning light goes on
Secondly, i higher fuel level may help the supply of fuel (better pressure/ fuel head/ fuel surge less liable etc)
Or it may just be a placibo effect
When i have a full tank, i can feel the extra weight slowing the car down, If you consider a full tank of fuel (Mondeo) weighs almost 50kgs (approx 8 stone- the weight of a supermodel) its not surprising
I watched the BTCC saloon car racing and i remember the chap talking about how much an extra 18kgs was going to slow his race car down, ("success" ballast is added to BTCC cars to slow them bown, upto 45kgs, i think)
I know our cars are not race cars but the same principals apply / weight saving is also a cornerstone of hypermiling (economy modifications and techniques)