New Pirelli webbing or cheaper but just as good rubberised fabric webbing available from upholstery shops. A quick and easy job and very satisfying. Webbing needs to be tighter than you might think. Undo the poppers on the seat squab and lift off. Seat belt webbing can be used but it obviously doesn't have any give but is basically free from any pick 'n pull. I've used it and it works well. If there is a big difference between you and your partners height you can help her out by fitting spacers under the seat rails. Use strip rather than big nuts, to keep the sliders straight. There is some height adjustment and it might currently be set at the lowest position, so worth checking that first.