How I smirked about bulb changes until a guy in the office this morning said my offside brake light was out. Changed it this evening after undoing first aid kit bracket, replaced stop/tail bulb, tested all bulbs, put bulb holder back into place, tested again. Oldest son said "One of the side lights is not on". Took holder out again - no secondary tail light bulb in holder. Bulb found to have dropped out inside lens housing. Tried mechanical fingers, magnet pen but to no avail. Finally achieved success with a blob of blu-tac. All working now and bracket refitted - minus screw nearest bulb holder.