All I'd say is that in response to a Freedom of Information question the Forestry Commission confirmed that they had seen what they believed were two big cats in the Forest of Dean whilst conducting deer counts by infra-red. This means that the FC believe that this is what their men saw. For me too many intelligent people, including a close friend of mine, have had definite sightings to dismiss the possibility. My friend saw what he was convinced was a cougar whilst stalking with me on the edge of Exmoor, the most interesting point was that when this was mentioned to the farmer he was quite matter of fact and said that they saw it once or twice a year.
I am however intrigued by a couple of things:
1. The sightings most frequently describe the cats as "black cougar" and these are almost completely unknown in the wild whereas black leopards (commonly called panthers) are more common that normal coloured leopards in some parts of the World (parts of SE Asia) and were kept as pets prior to the Dangerous Animals Act. It is not beyond the bounds of possibility that some of these were released as a consequence of the act and would have easily been able to survive in the UK given the amount of deer and livestock in most areas. To be a black cougar there would probably have to be a hybridisation between a cougar and a panther and I don't know whether this is biologically possible or not. Many big cats can interbreed but again are the offspring fertile? It should also be remembered that even where these animals exist naturally and are relatively common it is still sometimes very hard to catch a sight of one and even experts can go many days or weeks without seeing one.
This brings me on to my second point:
If, as is generally assumed, the cats originated from pets released as a consequence of the Dangerous Animals Act, they would have to have been successful in breeding for a number of generations and this obviously requires a male and female of the same species to meet up. These animals have huge territories and it is well known that males roam extensively to find a mate and again it is not beyond the bounds of possibility that this could happen but is more likely to have been cougar than leopard/panther as it seems that more cougars were released. If a cougar and leopard can produce viable offspring then a "black cougar" has a clear explanation and also once a litter had been produced then the chances of further breeding in that area, albeit inter-breeding, are substantially increased.
There is fairly strong evidence that cougar, black panther, and lynx were released in the 1970s (and one or two other smaller species) but what is not known is their capacity to reproduce and still be there today. I have not seen one myself but I am not prepared to call those that are convinced that they have liars or idiots as there is no reason why they should not be out there.