They are for pulling a single wire, not boundary net clamps. If you're going to pull 300+ metres, you'll need the correct tools.Get one of these:
http://www.ebay.co.uk/itm/Hausen-Ch...-Fence-Tensioner-Pull-Stretcher-/311873513463
And before anybody tells you they're no good, I've had Hayes and these. The Hayes are better, but not £100 better. And it's not like you're off contracting with it.
With the cost of materials for fencing being so high, you either do the job properly, or pay someone else to do it.
That fact that you're recommending the wrong tools for the job indicates that you don't know how to do the job properly.
The problem with the cheap copies is the clamp. The profile isn't great and they will slip after digging into and damaging the wire. I too have both these and Hayes strainers (for single wire) and I'm a farmer not a contractor. The difference in cost of decent tools which will last a lifetime of use in your example is less than the labour to erect 50m of fencing.
You pays your money, but only a fool throws good money after bad.