Halfords cut my brake sensor electrics

[Sas.]

Hi there, I took my car into Halfords auto centre for new pads and discs. On collecting my vehicle I was informed that a mechanic cut my sensor wires because the warning light wouldn't turn off, how he done me a favour because I will come back year after year for free brakes not a problem. Immediately I knew this was a no no.. going back and forth with Halfords who are now lying about everything and have requested a diagnostic check which I did and it shows not only have they fitted the wrong brakes they have indeed cut my sensor wires but are claiming no liability for a new made up lie they are now telling. What do I do now
How do I start the court process
Is there any help for court applications on a low income
or a no win no fee legal firm that could help

thanks
sas