Coolant leak all over my leg mid-ride

[treeno]

So while riding yesterday I thought my leg had caught fire, turned out that hot coolant (a lot of it, the whole left side of the bike was dripping and now I can't see any coolant in the reservoir) had leaked from somewhere and most of it had ended up on my lower left leg/foot. I was only a few hundred metres from home and it was downhill so I just rolled it back. Anyway, my first thought was some cracked hose/loose connection so I was just gonna fill it and run it at idle and have a look. I've Googled and found mention of blown head gaskets which is slightly worrying... Do you guys think it might be that or will I be fine to start it up and watch for the leak? Thanks!
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Current ride: Ninja 500R

[Rogerborg]

Step 1: I'd take the opportunity to fully drain the system at the water pump on the lower RHS of the engine. Flush it through with a hose from the pipe at the reservoir above the radiator until it runs clear, and keep an eye out for water coming out anywhere other than the drain plug.

Step 2: entirely depends on the results of step 1.
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Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[treeno]

So I filled it up and being daytime I quickly noticed it was coming out of a small drain hole on the side of the cylinder head (drains the spark plug recess). Turns out the cooling hose that enters the valve rocker cover by the spark plugs had worked loose. Now just need to find an o-ring as all the ones I have are either too thin or too thick!
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Current ride: Ninja 500R