Case C-310/11: Judgment of the Court (Second Chamber) of 19 December 2012 (request for a preliminary ruling from the First-tier Tribunal (Tax Chamber) — United Kingdom) — Grattan plc v The Commissioners for Her Majesty’s Revenue & Customs (Taxation — VAT — Second Directive 67/228/EEC — Article 8(a) — Sixth Directive 77/388/EEC — Supply of goods — Basis of assessment — Commission paid by a mail order company to its agent — Purchases by third-party customers — Price reduction after the chargeable event — Direct effect)