Contract law please help

independant

I am an sales representative. Been employed for 11 years with my current employer. My latest contract of employment was signed in 2010. The terms of this contract and that of previous ones state that receiving upheld complaints can result in the representative's discretionary bonus being reduced. The company retains the right to recover cost incurred as a result of any complaint where this has resulted from the actions of the representative.
The company have received a complaint recently which dates back to business I set up 7 years ago. The complaint has been upheld against me (by the company). The company have confirmed in writing to me that the cost of rectification will be deducted from my future business production related discretionary bonus payments. I estimate that the cost of rectification will be around £20,000 to the company and therefore to me.


My question is can my employer legally apply a financial reduction to my bonus payments to cover the cost of rectifying a mistake I made 7 years ago whilst working for them under a separate contract of employment?


Any assistance would be very much appreciated