Thanks for the suggestion but that has no effect on them being grayed out or not. When this problem occurs it does not matter if the Mask is highlighted OR the adjustment itself.
But thanks anyway.
I am currently without the issue on the above example having removed the problem by adding additional adjustment layers and removing them again. Even if I now select just the adjustment mask I can still access ALL adjustment controls without problem.
But it will occur again as it has done over the last few months on a random basis. Hopefully someone else has seen this occur and found a solution?