Year: 2005
Description of problem (please be as in depth as possible):
Hi guys....I'm getting very frustrated with my lovely 2005 Cooper

I bought it from a Mini dealership at the start of October 2008, and just before Christmas, I noticed that the coolant reservoir was empty. I took it to the garage and they said there was a problem with the thermostat casing, and they replaced it.
About 6 weeks later, I checked again, and the reservoir was empty again. I took it back, they said there was a leak in the radiator, and that they had replaced it (all this was done under warranty).
I have looked again tonight - about 4 weeks later - and the reservoir is below minimum level AGAIN I have checked the oil - there's no milky residue, and there's no oil in the coolant, so I don't think (hope it isn't) the head gasket.
Has anyone got any advice about what they think the problem might be - and how I can get Mini to fix it properly??

Any suggestions very gratefully received!!
Any non-standard parts that might be involved with the problem?