Starter motor problems....?

I'm having intermittent problems with my starter motor / solenoid. When trying to restart the car after approx 1hr, having driven for some time in hot weather and heavy traffic, the starter motor simply clicks (solenoid sticking?) and the motor doesn’t turn. Sometimes after a few attempts it eventually turns and the car starts. On a couple of occasions the only way to get going is to push start…

Having left the car for a few mins or few hours or from cold there are no problems.

Am I looking at a new starter motor or just solenoid? Would taking the motor out and spraying with WD 40 help cure the problem?

Cheers - JK