Mitsubishi Lancer Sportback

Recently I have submitted my mitsubishi lancer sportback for it's first MOT, and has failed due to the rear middle seat belt. As the car is now over 3 years old the warranty has also expired.
I spoke with my local mitsubishi dealer and was quoted £160 for parts and £40 per hour for labour!
Unfortunately for me the warranty had expired by no more than 2 weeks, but the fault must have developed before that as that seat belt is never used (rarely have passengers in the rear, not a case of disregard for safety).
My question is, is there a way to get this seatbelt fixed (and pass the mot) without expense to myself, or am I going to have to grin and bear handing over £200 for 1 x seatbelt?
I have read online that seat belts are subject to a lifetime guarantee, however digging a bit deeper It seems only to apply to the USA and not the UK.
Any help or advice much appreciated.

Mark

Apologies, I should have stated that the seatbelt has locked (from my understanding the retaining pin used in a crash has locked the inertia real into position), resulting in me being unable to pull out the seatbelt.
I've taken a look at it the best I can, however the inertia reel itself is embedded into the rear seat. I've stripped the seat best I can, however I still can not get full access to remove the plastic casing for the inertia reel to reset the seat belt.