Many thanks for your patience, I am pleased to be able to continue assisting with your query now. First of all, I am sorry to hear about the issues you have experienced in your situation.
Whilst the clause in question can potentially allow the employer to charge you for actual and genuine losses incurred as a result of your breach of contract by not working your notice period, it is very important that they are able to justify such costs and show that they were genuinely incurred and were unavoidable. Most of the time, when an employee leaves without notice, there will be no immediate or serious costs to the business, apart from inconvenience, especially as a lot of the work can be taken on by others or delayed/postponed.
To be able to charge you anything, the employer must be able to show that the work which you were supposed to do during that notice period was not possible to be distributed or taken on by others, or postponed, that whatever happened as a result has clearly cost the company money (it must be costs which have actually been incurred, not just inconvenience) and that there were no other more effective ways to deal with it, meaning these costs were unavoidable.
If you do not believe they will be able to justify the above, you can indeed consider pursuing them for unlawful deduction of wages. To take the matter further, the employer should be contacted in writing, advised that their actions are treated as unlawful deduction from wages and/or breach of contract and request that they repay the amount in question within 7 days. State that if they fail to do so, legal proceedings could follow to recover what has been unlawfully deducted.
If the employer does not return the money as requested, the following options are available to take things further:
1. Employment Tribunal - the time limit to claim is only 3 months from the date the last of the deductions were made. Before making the claim, ACAS must be engaged to negotiate with the employer and try to resolve this without the need for legal action. The next steps to initiate this procedure are to contact them, either online by filling in the following form (https://tell.acas.org.uk/find-a-solution-to-your-employment-dispute), or by phone on 0300(###) ###-####
2. County Court – this is an alternative way to claim and the advantage is that the time limit is a much longer 6 years. It is therefore a useful alternative if the 3-month deadline for the Employment Tribunal has been missed. Also, there is no need to negotiate before starting a claim and the claim can also be made online by going to: https://www.moneyclaim.gov.uk/web/mcol/welcome
Ideally, by warning the employer that their actions are unlawful and that legal action is considered, they will be prompted to try and resolve this and return the money with the need to take things further.