Hi,
Cashbacks that can be categorised as inducements, to an individual who is not running a business, to enter into a transaction for the purchase of goods, investments or services, and are received as a direct consequence of having entered into that transaction, are not taxable. Based on the information you have provided, the cashback sum you received from the property developer is not taxable. You can see guidance here:
Statement of Practice 4 (1997)
The following guidlines also outlines that cashback, in the terms outlined, are not a capital sum and not liable to Capital Gains Tax:
CG13027 - Incentive payments/financial institutions: cashbacks: contractual rights
Thank you.