The value of a supply is crucial for VAT.
The area of discounts has always been problematic, particularly in respect of prompt payments discounts (which customers sometimes do, and sometimes do not take advantage of).
HMRC has clarified the position and has published: ttps://www.gov.uk/government/uploads/system/uploads/attachment_data/file/320780/VAT_prompt_payment_consultation.pdf … which confirms that from 1 April 2015, all businesses that offer Prompt Payment Discounts will be obliged to account for VAT on the sum actually received in return for the goods or services supplied (rather than the reduced value offered – as is the current position).