You could attempt litigation if you are game. Could be expensive.
If deposit is not paid back you could take legal action. It is usually held on trust for you until completion.
Serving notices is not too technical. It may be done in accordance with what is agreed in the contract.
You would have to read the contract to see what you have agreed on. You would normally give them notice to complete and then terminate the contract if they don't settle in the required time. You woukd then get the deposit back.
