Help Mazda3 !!!

Hello everyone!
I have a Mazda 3, Diesel 1.6 Model 2003 ... It has been giving us problems for a while because being running as it is "drowning" until it stops ... sometimes it starts again the first one but on other occasions it does not do it ...
we are living in the United Kingdom and the first time it happened to us and that we did not start, we called a mechanic that told us that it could be the solenoid valve ... He played in a kind of clamp (forgive I am nothing on the subject of mechanics and I only try to be graphic) ... it disconnected it, connected it again and the car started ... since then the times that has failed we have done the same and the car starts ... but of course, it is not the solution and we will know if someone can give us some idea of ​​how much that piece costs
thanks