How many miles to the gallon can a 1.9 diesel peugeot 206 do?

Depends how hard you thrash it! If you're really careful 40-45mpg should be possible.

If you're concerned about mpg, get the newer euro IV 1.4, it is a much more economical engine than the old 1.9. Or for a little more power the 1.6 (110bhp) is very nice. They're both newer engines, so might be out of your budget though.

around 50 or 60 miles to the galion

l get 530 miles from a full tank(53 litres to refill) in my 306 with the same 1.9td engine as yours, that is 45mpg!, you should be able to get nearer to 600 miles from your 206!
hope this helps

Considering you will likely be pushing it most of the time, about 100 mpg.