Coolant leaking Exapansions Tanks & Temp Sensor

Ok folks after a couple of head scratching and frustrating months I think I have found the source of my coolant leak. The flashing lights of doom on the way home from work prompted me to last night top up the header tanks and bleed the system yet again.  All seemed fine running it whilst the heater was on hot it was at the correct level so i screwed the cap on and ran it again to build up presssure and switch between hot and cold settings on the heater control just to test thermostat, heater control valve and pipes etc.  Did the obvious checks fan kicks in at correct temp andleft it running for 20 mins with no leak from any pipe or tank etc.  This morning after my drive to work, noticed a puddle in car park dirctly under where the header tank is…..I opened bonnet low and behold coolant everywhere and seemingly escaping from the top of the header where the sensor sits and throwing it out of the little channel at the side. My guess is either the seal or the temp sensor that s screwed into the tank isnt sealing ? are these hard to replace or re seal? and which is it the seal or the sensor that needs replacing…..?  Im hoping the thread the sensor fits into hasnt failed?

Any ideas folks….

if its a non-genuine tank that could be why, i bought a febi one which was crap, ended up putting the original one back on! sometimes the pattern sensors leak thru the pins, again bin and buy genuine if possible

OK the header is stamped with VW part number and says its vw part, do you think the sensor is leaking then or an o-ring that should be sealing the temp sensor?

could be either its hard to say, if you look closely hopefully the reasdon should be obvious. to start with i would unscrew the sensor so you can inspect it, the seal plus the tank to make sure its not cracked around the thread

tar jon will check tonight when cold

arrrg how anoying, topped up a bit last night and throwing out more than ever while driving round with heater on cold....why....