This problem has been on and off over the past year or so but the recent bout of hot weather in the UK with a temporary longer commute has made this problem arise again.

From time to time I notice my coolant level dropping; upon topping it up there is a slow dripping leak somewhere close to the coolant reservoir but it doesn't appear to be coming from there. As I usually reside up in Scotland and make short trips, it generally doesn't need topping up for sometime afterwards. However the past 10 days and for the following 2 more days, I'm doing a much lengthier commute (approx. 50 miles each way) and I've noticed that the car is going through coolant much faster. Last night on the commute back the engine temp. warning light came on and being unable to stop, I opened the windows, set the heater to max and blasted as much hot air as possible - this caused the light to switch off within seconds.

Today I did the 50 mile commute this morning after topping up the coolant this morning; checked it before the return journey and it seemed like the coolant level hadn't dipped. However after arriving back home I've noticed that the coolant in the reservoir is bubbling and there's a more substantial coolant leak.

Any ideas? 
 

Edit: Should also mention that I’ve not noticed any creamy fluid on the oil dipstick or oil cap. Left it for 30 minutes and now the reservoir is practically empty of coolant. 

The plastic expansion tanks are well known for developing lots of hairline cracks especially in the bottom section. When the system is hot and under pressure coolant is forced out. Lots of cheap new replacement tanks to be found on eBay.

The coolant bubbling can be caused by a few things, but often a sign that the head gasket is on it's way out. Buy a hydrocarbon test kit from Amazon or eBay for less than £20 and check for any exhaust gases entering the coolant.