FTF @ 19:20
I backtracked up the hill as suggested and followed the path towards the co-ords for the bonus cache. I then made the mistake of going to the right on the path down the hill and watching the GPS needle swing 100+ feet to the left. I'd missed the fork in the path in the darkness - doh!
I worked my way back and then found where I was supposed to go and found the cache after initially barking up thee wrong tree.
I then followed the path back towards the Jeep. This took me past the main museum building which still had a couple of lights on even though it was totally dark by now. I quickly made my way onto the road as I was not sure I was supposed to be in the grounds after dark.
Thanks for a fun mini-series of caches in a nice location!