Starting problems

Not all the time but occasionally we have problems starting the disco. I go to start it wait for the glow plug light to go out but when i turn the key there is nothing apart from a slight tick noise every time you turn the key which appears to come from the direction of the passenger side. There is no red key symbol on the dash. It can take two minutes or an hour before it will start and then it can go for days before it happens again.

sounds like the solenoid is kicking in - but starter not turning - remove it and check it across the battery.

next time it happens hit the starter with a hammer a few times then try it, if it starts yer need a new starter motor

we have tried that could it be the immobiliser the clicking noise comes from the starter relay or from the black box by the fuse box