Fatty B is correct - a bodyshop job, and you'll prolly need a new spoiler too - I did!
I dont think its a legal requirement, as you still get old cars on the road that dont have it......
Then again, it may be a legal requirement for all cars manufactured after a certain date.
Nevertheless, I would get it sorted TBH. It would bug me otherwise!
I'm not 100% sure on the MoT side. However, if it has it fitted, it must be working. A non-functioning 3rd brake light would mean the car fails to comply with the "Construction and Use Regulations", because the car would no longer comply with its original Type Approval (look at the bottom of your left B pillar for the aproval number).