Bought an 03 plate, streetka back in September. Had short mot, but flew through. Not long after heating only blew cold air out. Checked the coolant. Was completely empty. Refilled and off it went. Fine for 2-3 months up until last week. Exhaust began randomly blowing. Turns out the rear box has snapped off. Then two days later, the temp warning light comes on.. Pulled up at work. Lifted bonnet. Didn't seem particularly hot, but could hear a sizzling sound. Panicked a little left it to cool and checked the coolant. Was none. So I filled her up. And drove home. Again. Temp light came on. But got it home. Left it to cool. Coolant had gone again. Topped it up. All the while hot air was still coming out. No steam or anything. Drove to work again. And then temp light came on, and I had to pull over this time I had steam gushing from under bonnet and I could her the engine like a cooking sound. Lifted bonnet and left it to cool. As I was only 5 mins from work . I carried on with my journey fine. And then left it to cool in car park. Went and checked on it. No coolant in tank again. And the hose has snapped off the heat valve. So I had green flag tow me home. Ordered the part and awaited to arrive. Fitted the part, think it would solve my answer. Had it go into garage for exhaust. And asked them to check for head gasket failure. (I'd already checked the oil cap and was clear ) they did and had no sign of over heating or hg failure. So assumed the heater valve had answered my issues. Drove it around for 10-15 mins. And again the engine began to over heat. I got it home again. And let it cool. Puzzled as to what it could now be. Took it back to garage next day.  Had them do a sniff test on water for the head gasket. And it came back clear. Nothing. But yet when I drive around with the heating on full blast it stays on for a while if I !Removed! off the heating it begins to over heat... Some one please help me. The garage are saying they don't know and can only assume the hg tests a wrong and it need replacing... They over quoted me..so I won't be going with them. I just can't except that it's the head gasket despite countless tests coming back clear. What is wrong with my car!!! 

Just need to add also... Have attempted to bleed of any air by letting it run st idle with the cap off. And squeezing the pipes. Checked the hose coming from the valve to the top of the radiator it was getting hot. The one coming out I didn't check while running the engine. But after I switched it off. It didn't feel like it had been used and felt pretty cool. So guess I'd have to check that again. I am to in and trough in from thermostat, to coolant tank, to head gasket, and to maybe another dodger valve. The fans a all kicking, there's nothing in the water. I just can not understand what is going on. If I drive for more than 10-15 mins and shut off the heaters, it begins to over heat, steam pisses out and I have to pull over and allow it to cool... Someone please help me. I can't give up on this car... It's such a lovey little thing. I just refuse to accept that dodgey mechanic telling me even though the tests are clear of a faulty hg, that he still thinks I should have it replaced at a ridiculous cost... 😩