Are you including the following command: fastboot erase userdata?Did factory data reset in stock recovery first before following fastboot flash procedure. Did my lads Vodafone UK branded phone to 5.1.1 EU Retail 3 days ago and its perfect.
Though this on which is Tesco Uk is not right. Done it again but it will not boot to recovery as just displays green android and No command so can't wipe in recovery now. Can't go back to 5.1.1 either as it fails on flashing boot probably due to MM partitioning is different. Still makes system hang if facebook is installed too.