On Saturday, 20 February 2016 18:22:56 UTC, Janet wrote:

I can't tell if there's any slope towards the colelction chamber; if
not I'm not sure it will drain the lawn much.

There's no slope. However, a sump (which is what this would be acting as) doesn't need one as it works on the self-levelling action of ground water. My issue though could be the non-permeability of clay which might reduce the amount of water that can travel towards the chamber - it is hard to say though as I don't know the relative heights of each layer.

I strongly recommend you ask some advice from Tony at his website
here
http://www.pavingexpert.com/

Yes, I know (of) Tony - he taught me everything I know, and then some, for several patios I've laid. I'd rather not send an unsolicited request to him though as I'm sure he gets inundated with them. Besides which, all his info is on his site and if I've got Tony right I'm pretty sure he'd equate my half-a-job drainage idea as being akin to the 5-blob patio laying method and lambast me accordingly!