David,
I have experienced exactly the same issue with a water leak in this area of my 3.7 whilst in the South of France a couple of weeks ago. For me it was a hair line crack in the white plastic tee piece. For your information the tee piece is an 18mm barb fitting. Make sure you have a new one before risking damage removing the old tee it could be fragile. I managed to source a Gardinia garden hose pipe Y piece from a French DIY store to get us home but this is not a long time fix. I intend to replace with a more substantial stainless steel barbed tee. Better to replace before it lets go completely if yours has a fracture.
Make sure you top up with the correct (red) anti-freeze once fixed.
Hope this helps.
James
