If you do not agree with a proposed deduction, you can raise it through the scheme's free dispute resolution service. You do not have to accept the landlord's figure, and you should not feel pressured to settle quickly.
An independent adjudicator compares the check-out evidence against the check-in inventory and decides what, if anything, is fair. The disputed part of the deposit is held by the scheme until the case is resolved, so it cannot be paid out while you disagree. Be specific and evidenced. Dated photos, the inventory and any cleaning receipts carry far more weight than a general claim that the property was left clean.