Volkswagen Polo staring problems when left in the sun

An odd problem that I've been monitoring for over a year now. The car doesn't start straight away, and it takes about 3-5 tries to get it started - and then it runs fine. However this only ever happens when it's been left in the sun, so it's very common and annoying through summer, but has never happened during the winter, or early mornings and in the evenings, or in the summer when I've driven the car a fair distance and only left it for about up to an hour that the problem won't happen, only on summer days when i've left it for over an hour :/

Any help or suggestions will be much appreciated.