By Josie Clarke, PA Consumer Affairs Correspondent & Lawrence Matheson Farmers are urging MPs to back stronger rules to shield them from “unfair” treatment by the major supermarkets. A demonstration involving dozens of ‘scarecrows’ outside Parliament is planned for Monday, as MPs prepare to discuss changes to the grocery supply chain following a petition signed by over 110,000 people. The petition, started by veg box company Riverford Organic, calls for regulations ensuring supermarkets stick to “fair” buying agreements. This includes purchasing agreed amounts and paying the agreed sum on time…