Intermittently Dead

Hi all, this about my Focus 2002 1.8 TDDI

When I sometimes park up and return to the car a little while later it is dead when I try to start the car. Leave it several hours or even overnight and it starts first time. I've tried earthing the engine block direct to the battery negative terminal.

All the lights on the dash light up when I switch on the ignition, radio, headlights and electric windows work but when I try to start it there is not even a click and as I mentioned, leave it for several hours or until the next day and it starts no problem. When it’s running, it runs perfectly.

Does this sound like a starter motor problem or any other suggestions if it’s not please?

Obviously something is happening when the engine gets warm…

Thanks