Ok thanks. You need to press her for more information on why that money was withheld and state that unless she can provide genuine proof of what it was for and that it was allowed, you will take it further and pursue her to recover it.
You can do that for free in the Employment Tribunal if needed. Before a person can make a claim in the employment tribunal, they would be required to participate in mandatory early conciliation through the Advisory Conciliation and Arbitration Service (ACAS).
The purpose of this process is to allow ACAS to mediate between the claimant and respondent to agree on an out of court settlement in order to avoid the need for legal action in tribunal. The respondent does not have to engage in these discussions, or if they do and the talks are unsuccessful, the claimant will be issued with a certificate allowing them to make a claim.
However, if a settlement is reached, the claimant would agree not to proceed with the claim in return for the agreed financial settlement. Other terms can also be agreed as part of the settlement, such as an agreed reference.
To initiate the conciliation procedure ACAS can be contacted online by filling in the following form (https://ec.acas.org.uk/Submission/SingleClaimantPage), or by phone on 0300 123 1100.
If the conciliation process was not successful and you then wanted to make a formal claim in tribunal, you can do so here: