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When its cool, take the cap off and give gently rev it a couple of times and have a mate look in the tank. If you see bubbles emerging then exhaust gases are getting in. I'd personally flush it anyway, its not going to be a bad thing and it'll get rid of any old deposits lurking in the system
Hints at it boiling or exhaust gases getting into it somehow (headgasket or cracked block) but thats at the extreme side of things. As you've not experienced overheating yet I'd take it for a spin and check it out personally, you can buy a tester that'll show up the presence of exhaust gases in...
Theres some numpty by me with a Saxo VTS whos got a sticker bombed splitter and it looks gash, just like this. Seems to be the in thing with the kiddies