Due to huge experience in this field, (correcting mistakes on my paintjobs on plastic kits) Mr Muscle oven cleaner does not seem to bother the clear parts on those. Spray it on parts, leave in an old ice cream tub sealed (or even a plastic bag) overnight and rinse off. Be sure to use gloves, though.
Fairy Power spray is now the product of chioce for this task, although I haven't got around to using it myself. No reports yet of it crazing clear Polystyrene in Airfix kits!
Cheers,
Roy