hey mate, i had a problem similar to this. mine was the key switch amplifier part that sits arounf the ignition barrel (should be brown). i pulled it off gave it a wipe and put it back on. started. i had to to this every now and then till it came more frequent. so i just replaced it.
its the part which send the signal of the key to ECU to say that this key belongs to this car basically.
that took me sooo long to figure out when it was happening to me. i used to get real frustrated that it wouldnt start when i needed it to. but would start when just going for a pointless drive.
let me know how you go. hope my problem helps