Case C-309/06: Judgment of the Court (Third Chamber) of 10 April 2008 (reference for a preliminary ruling from the House of Lords — United Kingdom) — Marks & Spencer plc v Her Majesty's Commissioners of Customs and Excise (Taxation — Sixth VAT Directive — Exemption with refund of tax paid at the preceding stage — Erroneous taxation at the standard rate — Right to zero rate — Entitlement to refund — Direct effect — General principles of Community law — Unjust enrichment)