ok, problem solved. i just downloaded decimation master from the pixologic download center again, even tho it is listed as 1.1 - which is same version i had installed before. The problem got solved!! pixologic and their dodgy version number updating :)
Painting masks for optimisation control = win ... that was one of the things I always liked about Lightwave's optimisation plugin. This should be great for taking things out just before baking (or 3d printing!) to keep file sizes and processing times down. I love Pixologic!