I would suggest that for each repair contractors must carry with them a freepost satisfaction card. The card has to be left with the resident to be filled after the contractor has finished and is gone. This is because I have known contractors intimidating residents in saying the job is satisfactory.
Apart for option of to say what they think of the experience, from reporting the repair to having the job carried out, the card should also have a freenumber to call if the resident prefers to call instead of mailing the card.
If you then do not hear from the resident or you do not get the card either you post them another card or you phone them up with a discreet inquiry.
This process should be done with any issue, not just repairs.