Strange coolant loss - found, I think!

[GS535i]

I've had a slow coolant loss over the last month, at the rate of ~1 liter/1000Km. No signs on rad, hoses, oil cap, or engine, so it started to look like the head gasket .
I did a cold pressure test - and it held 1 bar for 5 hrs: repeated today on a fully warmed engine, and finally the pressure started to fall - and a small pool of coolant formed under the firewall.
It appears that the expansion tank is the leaker, dribbling down the center mounting foot. It is not leaking from the overflow, cap, level sensor, or hose connection: must be a failed seam in the tank. Much preferable to doing a head gasket, but seems to be an unusual failure ?? ...So a new tank is in order - any recommendations as to the better place to find one ( also a 1.4 bar cap to replace the 2.0 bar one the dealer sent me previously).

[winfred]

m30 puke tanks dieing are varly common around here as they get older

[Patrick]

http://www.bmaautoparts.com/

[genphreak]

Mine went at the hose connection- people tighten them too much. Seams are common too depending on stress. New tanks have a aluminium tube inserted into the pipe connectro to strengthen it under the hose clamp area. Mine was sub-$50USD from www.eap4parts.com. These guys rock hard. They can get anything for you, are cheap and very good. GP