| There's a seam in the plastic which is the weak point in the coolant tank.
The only problem is that when you're riding the bike the air comes in and forces the coolant up and out at the top, so it looks like the leak is coming from the neck of the tank.
You may be able to do a plastic weld on the tank, or use a temperature controlled soldering iron with a wide flat tip and run that down the seam, but it's probably best just to replace it ... I think they're about 20 quid from a dealer? | 
antonye
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