coolant overflow problem

took the rs out of the garage today and let it tick over for ten minutes or so like i do every week and coolant started to come out of the overflow bottle by the battery, this is the third time its after happening in the past 2 months
anyone any ideas what the problem could be??

i would say it could be the thermostat stuck closed so coolant is only circulating the heating cuircuit not the whole system so it gets hotter and expands and flows into the bottle,

nice 1 dec