I know what you're thinking; 2.0 TDI with a BKD engine, it could be the dreaded porous head. But mine is a 2007 with a code C cylinder head. As far as I understand (and from what the Audi parts guy said when I was buying coolant) the porous head issue only affected early 2.0 TDI engines with an A or (less commonly) B cylinder head code. It's lost about a litre over the last year, but now it's lost about an inch this week (200 miles) from the expansion tank. It starts and runs fine though. I can't see any water where I park, but the undertray and never ending rain might be masking it. I thought maybe a faulty tank cap could be leaking vapour, but the Audi parts guy said he'd never sold any of these so it's unlikely. Is there any way I can test this, or is this an unlikely culprit? I also thought it could be the EGR cooler as this seemed common after a bit of Googling, but I remember Nige saying once that these aren't usually the cause, so I want to avoid spending £350 on something that's not faulty. Apparently another common one is the radiator, but I don't think there's anything dripping and I don't know which bit is the usual spot for leaks. I thought even a tiny pin prick hole in a rad would cause a jet of water to spray out. Is a leak of this quantity likely to leave a puddle on the floor, or would it just evaporate? Can anyone suggest what's the most common cause of leaks so I can narrow down my searching. It's driving me crazy, so if anyone can suggest anything I'd be really grateful.