I wondered if
this was you the other day after you mentioned your unit, but I can't remember why when I found it. Don't worry I'm having a somewhat confused day
You seem to have done well with technique. One thing I would try is a more aggresive pulse - I can get George to under 20 MPG sometimes on a pulse although the whole P&G thing is still experimental for me. A harder "pulse" may work better for you though in the heavier 318 which will keep some momentum - I think the Aygo loses too much in the glide to make it worthwhile but I'm trying to see if I can get it to work for longer journeys.
For mods I found had the most effect, I would go with :
1. Take your tyres to 45.
And then check the ride and ease back if it's too crashy - mine was. Get the alignment done as well just in case.
2. An outside block, clear plastic perhaps, for the Kidney part of the front grill (the upper part). I can send you the bolts from my bought in block if you want to PM me an address. That way you can just fix the plastic to them. There are only two though, you may need more depending on the material used. The lower one maybe after you've monitored temps.
3. Flat-mate tyre inflator and an AA card to save weight. This made quite a difference to the weight.
4. Leave off the rear wing, its for show
And these ones I never did but seem to work for others
1. Belly pan seems like an idea as it is very open down there where the extra 2 cylinders should be
.The rear end parachute is worth a look too whilst its up on the lift.
2. Something to hide the wipers from the airflow.
3. You could try the "Pizzapan" covers for the wheels, I'm not sure what benefit there is to be had there but some here use them.
4. Skinnier or taller wheels and tyres - maybe some steelies from a 5, or maybe borrow the wheels from your 535 to raise the gearing a little ?
5. MFree mirrors (or their cheaper Corsavan equivalents).
I'm sure there is plenty more to be had from this car though. Are there many options for chip-tuning a NA petrol ? The one guy I have checked with for Aygos says the whole idea is carp.
EDIT - PS I am joking about the engine, you can get George's into a shoe box