A proposed deposit deduction is a claim, not a decision. The landlord must support their entitlement; the deposit remains the tenant's money unless an agreed or established claim justifies a deduction.
You may meet claims for cleaning, damage, missing items or rent. For each, identify the obligation, what allegedly happened, the evidence and how the amount was reached. A specific price or forceful email is not enough by itself.
The money may be held by a custodial scheme or under insured protection. Use the actual scheme's return process; do not assume it has already received the funds.