Coolant escaping from under the cap on the expansion tank, I need advice.

[redback]

So i fired up the Ryker on the way back from work and noticed a strange low almost hissing noise.

After a cautious ride home and removing the bonnet panel i immediately noticed Coolant leaking out from under the cap on the reservoir and down onto the kick plate.

The coolant wasn't boiling off and the temp gauge was fine, so it's not overheating and there was no signs of leaks anywhere else. There's no visible smoke coming from the exhaust so assume it's not the water jacket.

It's too hot to check the cap right now, what do i look for when i check the cap?

If it's not a dodgy cap, how do i go about hunting down the leak and how big of job is fixing it?