I thought the 4x50gms I had originally bought might be enough to make this bag, but I ended up using twice as much and it still wasn’t quite as big as the given measurements before felting!
The felting process went very well though in my ordinary front loader washing machine on a 40deg cotton cycle and I’m pleased with the finished result.
I put a large towel inside the bag along with a couple of strong plastic tubs to help the felting process, instead of the suggested three tennis balls, as I didn’t have any of those to hand.