Well, replaced the drive with a new LiteOn LH-18A1P and low and behold, the problem remained - so I guess my son has finally got himself a DVD burner for his PC.
Good news is I finally got everything working again fine after removing all burning software (Alcohol 120%, Nero, and ImgBurn), uninstalling all optical drives, rebooting and installing only ImgBurn. All I can guess is that something got screwed in O/S during a recent crash and uninstalling Alcohol or Nero has reset it (I'd already uninstalled and reinstalled ImgBurn as it was the only burning app that had been used or changed recently - upgrading to 2.2.0.0). I proved to myself that the O/S wasnt hiccuping at ImgBurn though by installing 2.1.0.0, making a successful burn on DVD+RW, then installing 2.2.0.0 and successfully burning another DVD+RW.
There should be an upside to this - my wife's LiteOn scans DL discs immensely better than the old Pioneer so I have high hopes for this one.