Ok,
This has me stumped.
One hot afternoon after driving around i went to start my car and nothing, the fans kick in(as in inside, not the engine bay ones) but there is no kick from the starter motor, nothing at all!
We left it for about 30 mins and came back and it fired straight away.
This was a couple of weeks ago.
Now yesterday it's done the same thing, on a hot day, waited awhile for the car to cool down and then it fired straight away.
Is there ANYTHING in the ECU or engine bay, like sensors or fuses that would blow or stop functioning if the engine bay was hot?
Now the temp gauge reads perfectly normal so i dont tihnk the engine is running hot, and there is plenty of coolant in the cooling system.
I'm stumped :(.

***EDIT***
Also forgot to mention that i dont think it's the battery as the stereo still works fine and pumps, all the lights come on the dash, and also its a fairly new starter motor.

My mums car does this sometimes. We will let it sit there for a while, and it starts right up again. Its probably a relay or solenoid ??

Could be just the inhibitor switch (assuming you've got an auto)

It could be the battery still.. stereo uses a small amount of current so that would still work. and the starter motor needs a fair bit to start the engine.

how old is it? if its more than 2 years old.. id say try replacing that or have it tested or something first .

Could be just the inhibitor switch (assuming you've got an auto)
Ummm what is that? where is it? how do i check?

I dont think it is the battery, as when you leave it for awhile it starts up fine, and drives fine, battery is only about 1 1/2 years old.
Anyone else have any ideas?

turn the key all the way.......

could be the battery, yur stereo would draw a bit of power and if its not charging up properly, (i.e, Short Trips) then your battery could die very quickly.

could be solenoid on the starter sticking, next time it does it, give the starter a bit of a knock with a spanner.

my car has started do do that now. at first when it happened i thought the car was in gear instead of P but it was on P so i jus moved the gear stick down then back to P and it started up fine. try that next time. as som1 mentioned before it could be the solonoide

But if it was the battery, wouldn't it just not start always? not come good after awhile? or does the heat affect the battery?
I've tried starting it in N or P, when i try starting in gear (which it wont do of course) none of the interior fans kick on or anything, so i know it recognises the car is in P or N.
Dad had a listen and reckoned the solenoid was going in and out fine, it just wasn't starting.

Can the starter motor overheat at all?

***EDIT***
Also forgot to mention that we tried knocking the starter motor with a long screw driver but that didn't do anything.

Dude, i'm stumped as you are on this one lol.

The only thing i could think of is check the wiring. Hot days cause things to expand. Maybe when this happens, somethings is shorting. It's a long shot, but it's the only thing i can come up with.

Are you part of the racv? if so, call em and get them to come out and check your battery. i had a similar problem with my old vs and it was just that there wasnt much charge left in the battery, and sometimes, it just gets drained.... but if you leave it sit for a while, it works again. when you are trying to start the car, make sure you have everything off, a/c, stereo, headlights, etc. so as not to put to much strain on your battery.

It could actually be the inhibitor switch. It may be slightly out of alignment, and the heat will cause expansion that MAY cause it to switch in. Try jiggling the gear lever.

(I couldn't start in Park, above 25 degrees, I had to start in Neutral)

same thing happened to me with my auto nissan pulsar.
at first i thought the inhibitor switch but then a mechanic gave me the answer
like yours mine would only do it when the motor was hot
leave it for 30-45 min and she would turn over
my mechanic told me to hit the starter motor body a couple of times and presto she would start.
my wife would turn the key and i would be lightly hitting it with my handy wheel brace until the starter motor kicked
apparently when the brushes are worn they stick when hot and by hitting the body it releases them and makes contact
try that and see how you go :)

Had a similar problem with my old Ford Falcon XE stationwagon. Only difference was that this would occur at random times, not when the engine was hot.

Check that there is good contact on both ends of the thick wire from Battery to Starter Motor, and that the wire is not damaged. As said, tapping the starter motor and jiggling the gear level would sometimes fix this problem. I also found that rocking the car, and pushing it forward alittle while in Neutral would solve the problem.

With my problem, the above solutions would eventually work, but not 100% of the time, so I made up a length of wire with alligator clips on both ends to manually start the car by hooking up one end to the small terminal on the starter motor, then I just touched the other end to the positive of the battery to start. - Hence bypassing the whole circuit that contains Neutral Safety switch etc.

It's not a good idea to do this long term and you really have to be careful, make sure the car is in park, and that it can't be bumped into gear. But it should be ok to get you out of trouble.

Had a problem like this with my TR,
what happended was the resistor on the side of the coil was worn, one of the plugs inside the resistor was about to break off and it was just at that point where it would work some times and not others.
Worth a look? Not really related to the engine being hot but.

hmmm the heat could just be a coincidence, i'll try it when i get back to melbourne.