Hi Rik and others, I have used cable ties on external commercial aerial
installations for some time. There are cable ties and cable ties. It is
possible to buy very cheap ones but in my experience these are not really
intended for exterior use, and are  NOT UV proof. Choose a good 'exterior'
grade of tie and you may even get 'UV-proof' in the spec.
We use black ones but I am not sure whether this affects the loss/insulation
properties, as we only use them for cleating up coax. In practice the black
ones seem to last longer than the white ones. I have seem these ties last 10
years high up a tall mast, and still have to be cut off when the cable was
replaced.
I don't know how you use these as insulators, but could I suggest 'doubling
up' so one tie takes the tension and a second (not under tension) acts as a
safety in case the first snaps. It might save a few FETs!! or in Dave's case
loading coils! The other problem is this sort of plastic doesn't like being
'creased' as thhis will reduce its strength considerably. So some attempt
must be made to limit the curvature at the ends, and stop it folding.
Cheers de Alan G3NYK