Nissan Micra trickling noise

does sound like that , try letting the engine tick over with the cap off the water get the engine nice and warm and put the heaters on so you can feel they are hot , you may notice air bubbles coming up to the expansion tank - make sure you take the cap off first or you could get a sharp rise in the water level

Following on from this question, the car has lost a lot of water since. The hot air blowers started to blow cold which triggered alarm bells. After topping it up and checking it cold the following day, when the radiator cap was removed, water spewed everywhere as if it was under pressure. My mechanic has checked for leaks and there are none apparent. The water pump is ok. After topping it up again, this time about an egg-cup full after a ten mile journey, it seems ok. The engine seems to be running ok and there is no water coming from the exhaust and no water in the oil. He said there may be a problem with the head gasket but is reluctant to look at the moment as this would mean a great expense and the car seems to be fine at the momnent. Do you think there is a leak inside the engine and am I right in thinking that the problem is still unsolved? If so, what would your advice be?