Playing devil's advocate, how do fixing folk's bikes as a favour play out regarding indemnity / liability - ?
If said person then had a crash which they then blame on you for 'messing with their bike', how do you fare - ?
AFAIK if you repair something ‘as a favour’, and it fails, you could be liable for the results even if no payment is accepted.
I test ride the bikes I fix, like to give them the opportunity to test the bike, and make the point (only half in jest) that anything done comes with my usual ‘30 second or 30 yard’ guarantee - as in 30 seconds after the door shuts or 30 yards down the road, whichever comes first.
No idea about the ‘legal standing’ of such an arrangement, but working on the standard of ‘reasonableness’, if someone did not want to pay for a professional job of work, could they reasonably expect a professional standard of work?