Well pretty much as the title suggests.

Had a little look in the header tank today and its full of thick black gunk. The car drives fine and has never had any problems with overheating, although since late last year from time to time the temperature warning light has come on but its done this even from cold starts in mid winter so i'm pretty sure it hasnt been overheating.

Head gasket gone??

Please see pictures below.

That aint good flush it and put new g12 coolant in

Why would it be filling up with black crap though? Its never done it before! Ive done 80,000 miles in the car with no problem and all of a sudden its full of crap!

If your eyes can cope WITH THE ENTIRE GUIDE BEING WRITTEN IN CAPITAL LETTERS, following this should help you diagnose if its head gasket failure.

The warning light is for coolant temperature and/or level, and the level sensor goes faulty on quite a lot of cars. If the sensor is faulty, you will get the flashing light and beeps even before you start the engine. The sensor is in the expansion tank, and looking at the state of yours, you'll be needing a new one anyway, so you can kill two birds with one stone.

My bet is that someone's put blue G11 coolant in with the pink G12 stuff...

How old is your car mate? Black gunk in the coolant tank can be a common problem - is there any coolant in there too or has it run dry? Temp warning light is most likely due to lack of coolant. Any traces of coolant in your oil? When was it last serviced? It probably ain't oil, more likely to be old scummy tap water, old coolant, mould possibly mixed with rubber from perishing radiator hoses. Flush that sucker out with distilled water a few times and replace with G12 as Banana Tobacco says and monitor it for the next 1000 odd miles.

Might help if I posted the link I alluded to yesterday!

http://reviews.ebay.co.uk/HOW-TO-TEST-FOR-A-FAILED-HEAD-GASKET_W0QQugidZ10000000001891247

As I say, my money's on some G11 finding its way in there...