Hello my name is ***** ***** I am happy to help you today. You can simply write to them to say you retract your acceptance and you do not intend to commence in January. As the term has not started there will be no notice applicable.
This kind of thing happens all the time so I would not worry about it too mucn.
If you have any further quesitons please ask. If I have answered your question I would be grateful if you would take the time to rate my answer. Thank you and all the best.
Even though it states you are in a contract you can only be sued for damages arising from the breach. If you let them know that you do not want to start whilst technically this may amount to a breach of contract the employer will not suffer any financial loss so cannot sue you.
You therefore do not need to worry about this.
Please let me know if there is anything further you would like to know about this.