£20 isn't bad when you realise what a complete b*****d this cable is to terminate.
Thats the reason I went to Lintone to get them done as bi-wire (cost a damn site more that £20 too !)
Van Den Hul recommend 'professional' termination - well,they would wouldn't they
Why not try just cutting the plugs off the 'commoned' end,bare the wires a little and use it like that to trial.If you're happy with the results then get them terminated?
Either way,best of luck mate.
EDITED : It was Lintone that originally terminated this cable so don't expect a better job !! (unless you meant better than Sevenoaks).
As to the length - it might be an idea to twist the pairs together (Van den Hul Recommend this) which will lose a bit of length - just something to think about before you go lopping 0.5M off