Surely any rim with that wouldn't be advertised as 'tubeless ready' though right?
There was another leak at 180° from the valve stem that I didn't get in the photo.
The valve base looked very well seated in the hole, and the rim tape was perfectly cut to the edges of the hole. That looked ok to me.
I've taken
@Mattyp's suggestion and put the tube back in (on top of another layer of rim tape) under the tyre and inflated to 40psi. I'll leave that for about 24hrs then give it another run tomorrow with the soapy water. If that fails, might have to buy some wider rim tape and better valves.
Thanks again all.