Common problem mate. The overflow pipe runs inside the tank and degrades over time. Have you got a metal or plastic tank?
I have just done this job on my metal tank. You need to buy some new hose and two new clamps. Take the tank off the bike and remove the fuel pump you will then be able to see the pipes inside the tank. Remove the two pipes and buy some new fuel hose of the same diameter, cut to length and reinstall with new clamps. Its probable worth getting a new oring for the fuel pump while thats out as well, about a fiver from aprilia. Refit fuel pump and tank and jobs a good un. Two hours max and shouldn't cost much more that a tenner.
