Busted

As you can see my garden chair is past its best. You could just buy another but. . . . . . I don't do things like that. I want to fix it.
Problem: source a suitable material for the job that will tolerate being left out in all weathers and last at least a few years.
You might think: No Prob
The little section of the seat where I started repairing ?8.00. multiply that by the rest of the seat - at least ?48
Then you start thinking about the chair back, and the other 3 chairs. . . . . .
Any suggestions??????????
The cord I was using is sash cord - just to try - not sure what's the best option