With used car warranties, check the wording. My VW Approved Used warranty only covers named components. With all the problems I've had with my car in the last 6 months, VW have only approved the change of 1 CV boot. One garage I worked at had a 14 day 100% warranty, the rest of the year was with them contributing only 25%.
The Tesco warranty does look really good, it does have some exclusions that raised my eyebrows though so do check the smallprint before committing.