There are Pine Martens in Wales and it would seem they are a genetically different race to the Scottish ones. See the BBC article click Pine Martens Wales
‘There is a small Welsh population. Pine marten scat found in Cwm Rheidol forest in 2007 confirmed using DNA testing. A male was found in 2012 as road-kill near Newtown, Powys. This was the first confirmed sighting in Wales of the species, living or dead, since 1971. ‘ ( Quote and photo from Wikipedia)
So what do I know about their existence in Ninewells wood.
Firstly this is an ancient woodland site so presumably was home to Pine Martens many years ago.
Secondly it has had pine trees growing on it for about 100 years or more and despite the felling of my Pines, there is still a large part with Corsican Pine and Larch growing at the present.
Thirdly my neighbour told me that she had met an elderly gentleman whilst she was taking her dogs for a walk who had told her that Pine Martens exist in these woods.
Fourth she also said that her other neighbour had seen a Pine Marten in their garden, which backs onto the woods, only a few days ago.
On the other hand, I spoke to a chap from the Deer Initiative at the recent CONFOR woodland show who has been putting up cameras all over the Wye Valley and Welsh borders and he has not seen any Pine Martens on any of the cameras including the one he put in Ninewells wood.
His view and mine is that it is possible that they are there but most unlikely and that it is fairly easy to confuse them with Mink which are definitely about. Below is a photo of an American Mink again taken from Wikipedia.