i had the same problem, and i replaced the heater valve, this cost �5 + vat from the scrap yard or you can find them on ebay for around �10. im no mechanic, but i was able to replace this in about an hour. 1st you need to remove the window wipers- just remove the nut from the bottom of the wiper and the wipers pull off. then take all the screws out of the grille at the bottom of the window screen then this just pulls off in 2 parts. then remove all the screws from the plastic housing that was under the grille, this again is in two parts. their was a lot of bits attached to this so i didnt remove it but i was able to move it enough to get access without stipping off all the attached bits. now you can see the bit you need to replace, it has 2 pipes at the front and 2 at the back with a metal cylinder to the side. using a pair of pliers pinch the clips and move them down the pipes, this will allow you to pull the pipes off tha valve, you will lose some water, but not that much.then just remove the valve by pulling it forward as it is only held in place with a hook. then put in your replacement valve and put everything back togeather. check your radiator water level to ensure the bit of water you lost hasnt taken it below the min level and then take your car for a drive with your heater switched on and this should get the water circulating round and hey presto a heater that works!!!!