Thanks to everyone that read my post, after much searching I think I've finally found some suitable cable.
I tried all the big electrical component suppliers, Digikey got the closest but they wanted an arm and a leg considering I only need a few meters of cable.
In the end I got back onto eBay, if you search for "DC Power Supply Extension Cable" you'll turn up pre-made cable with 2.1mm jacks on the end. A ten meter cable was about £4 so I'll just cut it down as I need it.
Cheers.