Hi Knot Board,
Yep, that is the stuff I was talking about, I did in fact get mine from Whitworths. The suggestion of sliding back the cover and cutting the core shorter is a great idea, although I certainly havent had a problem with the 4mm stuff... (spectraspeed?)
Landyacht: Spectra isn't Kevlar... quite different stuff. Spectra is a polyethylene fibre, Kevlar is an aramid... (dont mean to split hairs!

)
The biggest problem with the Kevlar is that it isnt much good in the sun and suffers from internal abrasion, it also wont melt.
Spectra on the other hand doenst have that problem and you can cut it with a hot-knife.