At lot of this depends upon how long they've had the cars. Assuming that they're recent sales then I'd remind you of your right to see for yourself that a fault exists. If they've already had work executed or are refusing to let you inspect the car then they're on their own.
Also, remember that customers aren't entitled to new for old replacements - if cleaning doesn't sort the Golf throttle body you may choose to use secondhand parts or ask them for a contribution to the new part.
I used to get one or two complaints every year who I'd brush off, however it's now every month & they always expect the earth even on old bangers. To be honest I normally take a hard line & tell them to go to Trading Standards/Small Claims Court/Solicitor/Police/Watchdog (or whatever rubbish they're spouting) and it normally stops them dead in their tracks.
Last month's complaint was a man who'd submitted his car for MOT because he'd owned it for just over 11 months. He alleged his car (a 10yr old CRV that last year I'd serviced & put on 4 tyres and discs & pads all round) had failed the MOT miserably on brakes, tyres, suspension & the Honda dealer wanted over £1000 to put right. If I didn't send him the money he's report me to TS - I told him I didn't believe him & to report me, to which I got the usual whining & his scrubber wife shouting in the background. After the call I checked the MOT database, 0 advisory & 1 failure - a droplink! That's customers for you.