Endless! At least you've found one of the leaks.
Do you have a part number for that Pierburg solenoid, Mike, in case an alternative can be found?
The PSA number is 1922V9 They are not horribly expensive, and given the magnitude of the leak, I will source one locally, either from an independent specialist or a BMW parts supplier.
Your dedication to the cause is above and beyond, Mike! It would have got punted by now if it was mine - but I suppose they're a bit harder to come by in that part of the world....
It was lucky that it seems to have started leaking right at the end of our 1650 km holiday. It then lost about a litre in 50 km around town.
The new solenoid is now fitted. Nearly a disaster because the virtually inaccessible and invisible torx fastener that holds it in started to deform and the driver would corkscrew out before breaking loose the screw.
With no other option apparent, I found that I could just squeeze my battery powered impact driver into the space, and it managed to loosen the screw, but it could have gone either way.
A late response I know. Being a cheapskate I bought a thermostat from China for my 2007 Peugeot 207 1.6 VTi. It appeared to be suitable for my vehicle: "Thermostat Housing Water Outlet Fit For Citroen C4 MINI R56 R55 Peugeot 207 208 308 508 2008 3008 1.4 1.6 9808646980 V764558080".
After I had fitted it I was somewhat taken back when the coolant I was pouring in was pouring out just as fast from the thermostat housing! Turns out that while my original had a blanked off pipe stub at the 7 o'clock position, the one I bought was fully functional! I should pay more attention to part numbers but I think some Chinese vendors have a hazy understanding of the differences between the Mini cooling system (expansion tank on LHS) and the PSA vehicles, (expansion tank on RHS, supplied from a T branch at the water pump end of the rear plastic pipe.). This confusion also extends to the rear coolant pipe, most of the ones I have bought don't have the extra T branch. I should not get too upset about this- I am now the local unofficial reseller of parts for Minis.
And just a comment on the need for the additional (electrical) control on the flow of coolant under heavy load in a cold engine, perhaps BMW remembered the problems the MGF/Freelander had with their original thermostats, that were later replaced with remote pressure-release units. And I'm assuming that like the Rover K series, the thermostats in the Prince engines work on the engine inlet supply, to better control the temperature of the coolant entering the engine?
For Peugeot 308 CC (2011) in general, do we have to worry about (1) heavy snow on the roof when we park outdoor and (2) raindrop leakage thru the roof under raining please?
What could be the preventive measures to those issues?
We are lucky here that we do not get any significant snow, but I don't think that it would be a problem, the roof is quite strong. I would not recommend operating the roof below freezing due to the risk of damaging the rubber seals.
We have not seen any water ingress through the roof seals, even in heavy rain. Because there are no gutters above the doors, water drips from the roof onto the seat when the door or window is opened when the car is wet.
The drain holes in the bottom of the doors are easily blocked (and difficult to locate), which can cause the doors to fill up with water, but you can hear it sloshing when you accelerate.
This car really is a true tale of woe 
