Fiesta Combi Starting Problem?

Hi,my fiesta diesel has an intermittent starting fault,after waiting for the heater light to go out I turn to START position and there is nothing at all no click from solenoid even,when it does start it will be ok for a week or just until I turn it off and try to restart,fitted exchange starter solenoid from scrappy first few tries-nothing no click and then it worked for a while (2 weeks)then packed up again.Have removed and cleaned all earth leads and connections to starter no joy,it's doing my head in,could it be the starter solenoid relay?Any help at all please(x reg).Thanks.