Thanks for your reply and ideas!
I should mention that we are in the Philippines where the mains water supply is not clean.
The water that comes out of this eight stage slow sand filter has passed though reed beds, activated carbon, pumice and then through an industrial strength UV lighting system. It will probably be far cleaner than the origional mains water that was used to shower in! So you can see why it would seem a waste to use it on the lawn
We'll also test this water on a routine basis to make sure it's always at a high standard.
If anyone can offer ways to improve the system, or perhaps even unforseen dangers of using it, please let me know!
Tara