Don't be so hard on yourself Sevendown, the people of St Tropez have other things to keep themselves occupied with than shaming tourists. The SoF in general sees a lot of different size and shapes, you'll hardly stand out. Do you have any height to you as a size 18 can be a very different thing at 5" vs. 5"11?
I'm assuming you're a bit of an apple shape now? Perhaps a long flowy dress which crosses over on your chest would be a flattering fit if you're okay with showing off your arms? I've attached some pics of a gown Adele wore some time back, not really for the dress code as that would be overkill but just to show the shape I'm referring to (albeit with sleeves). Black and muddy taupes accented with cream, oatmeal, or white are usually safe colours for the upscale Mediterranean destinations, especially in linen and washed silk fabrics. Good leather strappy sandals, I'd recommend a hat if you're able to transport that easily as the sun is pretty punishing down there.
Rosie a mid season jacket is actually one of my items missing from my wardrobe atm. I've been getting round it by layering over a chambray shirt or utilising blazers. It's a bit of a stop gap measure though. I used to rely on a red peacoat in gabardine cotton from Comptoir des Cottoniers but it's just too big for me now but it was kind of ideal as it could also withstand a light rainshower. There isn't much in the shops I've seen so far. A light colour leather jacket (cognac, tan, that sort of territory) can also be good but perhaps that's too warm? As I'm on a bike daily I need something a touch warmer on account of the wind.
I like the new sandals and jeans Elle, though it's probably the angle of the picture, I prefer the hem length on the left to the right, though they're probably the same 
Have you had any success with folding it over, even just the once, not a full rollover? A trainer might need a touch more flesh in between?