Lilacben:
I've quoted, above, from the actual document that MOT testers have to work to. It's up to the tester to decide whether indicator lights are "inoperative, excessively damaged, deteriorated or obscured so that they are not visible from a reasonable distance". If so, they must fail the vehicle.
If the indicator light, as a whole, is missing, the vehicle must also be failed. Further, if any white light is visible from indicator, the vehicle will be failed. (Only orange light must be visible).
If the indicator housing has been taped up, then the final part of my previous post applies:
"An effective proprietary repair (e.g. lens repair tape, etc.) must be assessed on its merits considering security, colour, light output and durability"
So if the plastic is cracked or broken, the car won't AUTOMATICALLY fail the MOT test, but it will do so if any of the rules [which I've quoted twice now] have actually been contravened.
Chris