Hi, car is at the garage as when I was checking the screenwash I noticed the expansion tank was a bit yucky, colour of the coolant seems to be less transparent and there is a load of gunk on the side of the expansion tank. Garage claims that coolant can break down and cause this and think a flush is required and a new expansion tank. However, this expansion tank was replaced by the same dealer 2 or 3 years ago (need to find the receipt) and coolant would have been changed as part of that. Looking at the gunky stuff I'm wondering if it's oil in the expansion tank - oil cooler seems to be a known issue on this car (insignia 1.8). Oil levels are fine and no mayo on the oil cap so looks like it could be oil mixing with the coolant. If coolant was to break down, which I've never seen, would it look like this?