As for the Marseille to Barcelona trip. I went direct from Barcelona TO Marseille with no problems (I was in a twin though). The authorities gave me no hassles at all when I got to Marseille but the route is easily flown on one tank of gas. You are from Barcelona so I don't have to remind you of all the restricted areas in the Gulfe du Lyon. I did my flight on a saturday morning so it was a little quieter.
Athens Aviation are in most Greek airports as ground handlers. The main office is at Athens Airport so call them and they can assist with many smaller airports.
If in France and going along the coastal route from San Remo - Monaco - Nice - Toulouse - Marseille, don't worry about a flight plan. Just give ATC a call well ahead of any airspace boundary. They are very good and very helicopter aware (they have hundreds flying along the route everyday). If landing at Nice, make sure you are aware of the procedures with the helipad area. They have two helipads and which one you use is dependant on the runway in use. Nice has offshore GPS reporting points so have those in your GPS prior to going in. Nice ATC is very helpful.
Summer is coming so get going before it starts getting crazy in the Med.
