I have recently done the same (enquired about re-inforcement).
The company is
www.tenax.co.uk
I calculated on their prices that to do a 600x20m grass runway the cost would be......................
Are you sitting down?
£58,500
Unless my maths ae incorrect this is based on £195 per 20x2m roll.
I'm sure a 20% discount could be negotiated which would make it a far more acceptable £46,800.....
It would be cheaper to put a lake in and buy a Maule with floats