diesel problem?

hi,i have a citroen berlingo 1.9 diesel wich i have recently purchased its well maintained,but before i bought it ,it was parked up for a while.any way the problem is when i purchased it i thought it was a bit sluggish,but put this down to needing a good run,after around 100 miles of use it came to a halt,a good few pumps of the primer got it going for a few hundred yards then the problem happened again.when priming the diesel through the system the primer pump didnt go solid but small traces of diesel where bubbling out the primer button.i was also told there is a filter somewhere in the tank wich may be blocked.question is where do i start,i dont want to have costly trial and errors.if anyone can help it would be much appreciated.