I ha?d a leak, too from the cap. And coolant pooled on passenger side near front tire. I replaced cap, and "BAM " !!! Problem solved. I hope your problem is that simple

 

BTW long overdue follow-up post.... Ended up replacing both the reservoir cap and the TBH (it was inexpensive so what the heck). I don't know which did the trick and don't really care--no coolant leak now. On to other issues--like why all the door locks unlock with the first push of the unlock button on the fob (and dash) but only the driver's door locks with any push of the lock button(s).

 

And we're back...the coolant leak and I... This time, there's an obvious puddle on the ground, but what wasn't obvious is where it was coming from, since the puddle started behind and inboard of the left front wheel. Naturally I first thought brake fluid, but I lifted the hood and noticed it was the expansion tank that showed a loss of fluid. And everything under the hood was dry. So I crawled under the car and saw coolant drops on the bottom of the body sill just inboard of the drivers door hinge area, seemingly coming from where the inner and outer body panels meet. Unfortunately I didn't have my phone with me to take a pic. The Disco is parked nose down and driver's side down on a shallow sloping driveway so I'm assuming the leak is somewhere inside the car and working its way to the lowest point in the interior, then finding a seam to exit. That would narrow it down to the heater's coolant feed and/or return pipe, heater matrix, or O-rings (or a combination of those), am I correct? How much of a chore are those to replace?

 

theres a write up on disco web on easily replacing the heater core. It I. Loves cutting the pipes and using hose instead. I did it on my last one in an afternoon. Just gotta remove the glove box and a few dash trim pieces to access it and use a metal
hand saw to cut the pipes.

 

Thanks tnrangerover. I may try that. First thing I guess is to remove enough stuff covering the heater & pipes/hoses to see where the leak is. A project for the next non-rainy day...

 

check the top of your rad as well