The act that you had accepted the new contract may make things more difficult for you to challenge. Whilst the potential charges were not discussed at the outset, once the new contract was issued you would have had two options – reject it and not be bound by it, or accept it and be bound by its terms, even if they go on to create liabilities which were not discussed with you before that.
So if you had clearly rejected the contract, your argument would have been that the employer had attempted to make a change to your terms and conditions to cover such charges but as you did not accept it then such terms would not be binding on you. However, if you had accepted the contract, knowing what the terms in it were, then they would have become legally binding on you, even if they created new liabilities. So the key was to reject it if you did not agree to it.
So a court would take this into account and this may affect your chances of successfully defending this. Of course that is not to say that they may decide you should not be liable for all of them but that is a decision only they can make at their own discretion so whether it will happen or not is impossible for me to say at this stage.
I'm sorry if this is not necessarily the answer you were hoping for, however I do have a duty to be honest and explain the law as it actually stands. This does mean delivering bad news from time to time. I hope you understand and would be happy to provide any further clarification if needed. If you are still satisfied with the level of service you have received I would be grateful if you could please take a second to leave a positive rating by selecting 3, 4 or 5 starts at the top of the page. Thank you