Introduction to Stamp Duty on shares and Stamp Duty Reserve Tax (SDRT): more about Stamp Duty and SDRT: Stamp Duty Reserve Tax - the relevant day
The principal charge to SDRT in FA86/S87 applies when a person (A) agrees with another person (B) to transfer chargeable securities for consideration in money or money’s worth. But it does not apply if there is an instrument of transfer which is either duly stamped or exempt from stamp duty (see STSM031170).
The charge arises on the ‘relevant day’, which is normally the day when the agreement is made. But if the agreement is conditional then the charge arises when the condition or conditions are satisfied (section 87(3)).