Found the answer. The registry entries above are reported by CCleaner are "Unused File Extensions". If you untick that Option under registry Issues it will be a temporary cure.
Note sure whether to blame Skype or CCleaner for this problem. Been using CCleaner in various versions for a while now to clean registry & although I usually back up deletions. have never had to re-instate any.
Incidentally the registry entries for this issue as shown by CCleaner don't have the full path so you won't immediately find them.
Richard