[Arran]

I can't speak for the cowl and faceplate on a Predicta but I have run into a number of post war plastic knobs that do much the same thing. What I have noticed is that if they are in a damp environment they get a build up of white residue, maybe it's mold, maybe not, and they tend to smell, if they are kept in a dry environment it doesn't seem to do that as much, such as the screwdriver handles I have. I've washed such knobs in a solution of trisodium phosphate and hot water, and the white stuff comes right off and the smell goes away, this is what I had around so there may be something better to try, also it can damage paint if the mix is strong enough. I don't know if Tenite II is the only plastic that does this as there are other knobs that seem to "mold" but they don't have the smell, so they were probably using more then one plant based plastic at the time.
  In terms of making new covers I think that polycarbonate would be a poor choice, it scratches very easily, it has almost zero resistance to U.V, I don't think it's any more resistant to cracking and breakage then the Tenite was (maybe less), and it's expensive. I've noticed that most plastics other then phenolics like Bakelite don't age very well, all things being equal Tennite II and cellulose acetate probably hold up better then most. I wonder if anyone has managed to get a hold of the Telstar people, they were making reproductions of Predicta TVs a few years ago, as colour sets, clearly they were making, or getting the plastic parts made somewhere?
Regards
Arran