You could try some k-seal. Works wonders for this exact problem.
Never heard of k-seal, is this similar to rad weld etc? ive heard a few horror stories about rad weld like it doing more harm than good and clogging up the coolant system.
Sounds a bit spurious but have you checked the level sensor in the expansion tank as Ive had a couple that leaked a little bit around the pins and have you replaced the cap as well (that was the reason for my level sensors leaking 
)
Yeah, replaced this a few months back when the coolant warning light was constantly on. TBH I changed all the usual suspects such as head gasket, tank cap, expansion tank, level sensor, oil cooler, a few split hoses. Im not overly worried now as its manageable coolant loss to what it has been of a dribble every week (250ish miles).
I havnt let it drop down much below the max line yet though so maybe the system is just finding its own level?
Or maybe im just just clutching at straws!
