UK Car Repair Questions? Ask a Mechanic for Answers ASAP
1st Check the resistance of the main positive lead that runs down to the motor as anything more than a few ohms and it should be replaced
also try substituting the battery for another as I've known a very flat battery to drag down the system voltage too far even with jump cables
one other thing to consider is that the immobiliser is faulty and preventing the starter from being enabled - however if the dash / ignition lights are on then this usually means that its off
If this is all OK then its possible the car needs a new starter motor
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