In reply to ilw:
What I meant about a combination of two readings was that your estimate of weight per metre contains two measurement errors*. Any error in your measurement of the overall weight will combine with any error in your measurement of the length, giving an overall uncertainty that's greater than the uncertainty in either measurement individually.
Repeating measurements and finding that they're consistent with each other is a good way of establishing that some kinds of error weren't too large but doesn't do anything to eliminate the possibility of a systematic error which consistently biases the results in a particular direction.
I assumed the weight per metre was your real gripe. If getting more length than you paid for is what's bothering you then it's an easy fix.
*Error isn't meant to imply that you did anything wrong. The best labs in the world don't make perfect measurements.