I've been fucking ZX-4'ed!!
Got home, coolant hissing out the cap. The coolant is at ambient temperature. Suspicion? Failed water pump, causing block overheating, causing head gasket failure!!
I am sure it's the head gasket - topped it up just now with coolant, started the engine, coolant level dropped immediately. Topped it up, then it started bubbling! Still stone cold, and you could see whitish smoke coming out of the bubbles!
Head gasket is completely and utterly shagged.