French Speeding Fines.

Hi everyone, 

Just wondering if anyone's experienced speeding fines when driving through France on their way to Spain.....? I'm not a great speeder and try to obey speed limits but, despite this, I've had three speeding penalties in the last four months which have been a complete surprise to me. 

I paid the first two immediately but the third one has simply p****d me off. I'm just wondering if I would get away with just not paying it. Any ideas or opinions would be welcome.

The car in question is Spanish registered but never had any issue with any British car...

Thanks

Ricardo