Just remembered about the ombudsman for garages - might be worth looking into
https://www.themotorombudsman.org/knowledge-base/what-should-i-do-if-im-unhappy-with-the-garages-work-on-my-car
All about reasonable due skill and care - which should be easy to prove they haven't done that
Don't know what would be more hassle - this or going to court.