Originally I thought in the un-labelled enclosure next to the Heck's Macaques, in the primate enclosures near the Wolves and Leopards. Judging from the guidebook, the only monkey species that fitted were the Guenons so I went with that.
However, a bit of Googling and confirming with Zootierliste makes me think that the monkey was actually a Samango Monkey instead.
Thanks for that - interesting as we saw Heck's in both sides - wonder if that's moved in the meantime.
One thing I forgot to mention is there is now an also unlabelled pair of Francois' Langurs in the run of cages at the entrance to the kitchen garden.