You need the likes of Ethel and co to help you with this one properly!! My understanding is that the handbook would not suffice as written notice and I think that your employers have acted unlawfully. I am not even sure that they are allowed to do this anyway, epecially if you did not actually lose money for the company. I own my own business and the employment advisors I use get very twitchy about any talk from me of witholding pay for any reason except absolute proof of theft by the employee. Even then I have to put it in writing to the employee before being allowed to take any money from wages. I think you should take further advice on this one as on the face of it, it sounds like you are being treated unfairly. Best of luck, I hope if you were genuinely guilty of gross misconduct, that you have learnt your lesson! Keep us informed of your progress.
Sue