Complex sentence: Universal Carriers were only lightly armoured on their fronts and sides, sufficient to protect to some degree from small arms, shrapnel and explosive blasts; but were open at the top leaving the occupants completely unprotected from about shoulder height upwards.
Split into simpler sentences: Universal Carriers were lightly armoured on their fronts and sides, sufficient to protect to some degree from small arms, shrapnel and explosive blasts. They were open at the top, leaving the occupants completely unprotected from shoulder height upwards.