It just had to happen....
I've been wanting to overhaul the entire cooling system but since I'm under the dealership's warranty (they're very good and true to their word) the dealership wouldn't replace any of the parts until it failed (despite my warnings and referencing to this forum!).
Well it did - spectacularly! Was cruising down the M5 to visit my in laws, and then boom, of a sudden - coolant level warning comes up. Pulled over and switched off engine immediately, grab my 5L of distilled water in the boot to top up but no need to! Coolant was dumped all over the road, definitely not a trickle. Chinese New Year, pregnant missus and 9 year old son, parents waiting, , and 36 degree heat = not fun!
Waited for a tow for a couple of hours whilst trying to organise who pays for what. All up, now car is in Canterbury BMW while they assess what needs to be replaced!
Moral of the story - change those expansion tanks!