Austis used plain steel wire which over time takes on an oxidised finish and settles into the bonnet rubber.
Could it be that modern 'rubber' is not as soft.I have never had a problem with bonnet wear using steel wire although some clips have taken on a 'bright' finish.I always clamp each end down with a pair of pliars or Moles.

Thanks for all the suggestions, will head off to my nearest garden centre on Saturday. Cheers! Hugh