hi. Please don't think I am daft but does my x-type expansion tank have an overflow?. I tried to top up my coolant as it showed below the minimum level and it leaked out via some kind of external channel on the side of the tank. Surely with a pressurised system this cannot be there??

They all have an overflow outlet. It's because when the systrm is hot and pressurised, the cap acts as a pressure relief valve and any excess coolant is allowed to leak past the pressure cap and out of the system to prevent over pressurising the system and causing something to pop. There is channel on the inside of the expansion tank where the excess will flow out. When filling the tank, it's easy to splash a bit down that opening.