Quick update – despite my cynicism, just telephoned Velux and they said I could have a spare part of bottom bar set with cords that they would send me. Cost £38 including VAT and postage and will be with me in a week's time. My blind was made in 2007 so I think it's had a fair innings before repair! So I don't really mind paying the money. Just glad it will be fixed.

Over a year later and exactly the same problem has happened to me. Noticed the cord fraid a couple of days ago and saw that the reason was the plastic had worn through and split. Lo and behold today, the blind jammed up as the cord had completely fraid through. Looking through this thread, I don't hold out much hope of getting a replacement part from velux.