I just want to let you all know I have managed to get my problem solved.
SLB, you were spot on - the Rx pair (which are on pins 3 & 6) had not been wired into the same twisted pair. I had tried to copy the colour sequence from an existing cable however had misread a couple of the bicolour lines in this cable because they were embedded in the connectors. I guess, also that I was happy to assume that the twisted pairs would run side by side, in turn into pins 1 to 8 of the RJ45 connector. Hence the DC charactoristics were fine but the noise immunity and transmission line charactoristics were totally wrong.
Once I read the messages today I went back and checked a pre-made working cable carefully (+ internet info' check). Hence the problem was found.
FL310, williamsg, --o-CK-o-- and of course SLB - thanks very much for helping me with this problem. It is a credit to yourselves and the spirit of the PPRUNE community that I was able to get this quality of help so readily. Our company IT dept didn't have much of a clue.
Fujiflyer