Nylon cord can be had from many places. B&Q, Camping and outdoor shops, Hardware shops. Try to take a sample along with you and have a good idea of how many metres you need - twice length of curtain rail plus twice the drop plus a bit more. Even John Lewis might be able to sell you the cord! Expect to pay as much as £10 - best to re-cord with the curtains removed and the rail on the floor - might take as long as 2 hours!
Take care when buying the cord - too thin and it will wear out quickly - too thick and it may not fit in the pulleys etc. After cutting ends - burn the nylon with a small flame to prevent fraying strands. Lighter 50p
Peccavi, October 2008