@DavidSWP While I understand your comments about being stitched up by a poor installation there area few points to bear in mind from a trade point of view.
1) Once the materials are at your house they are (generally) at your risk
2) Once they are fixed in place an installer cannot remove them (legally)
3) On a larger job, the financial exposure to a poor customer can be significant and irrecoverable
4) Minor jobs are the one's where no up-front payment is needed - low risk
5) Materials have a value and are the customers property once paid for
I suspect that we've (trade) all had a few bad experiences with slow-paying customers - usually the one's who want everything done as quickly as possible but seem to be slow finding card/cheque when the jobs finished.