OK, just following up to let folk know how I did this. I couldn't find an ignition switched fuse in the passenger footwell so I routed the dashcam wire down the passenger side windscreen trim, out through the body in front of the door and into the engine bay. I piggybacked the rear windscreen wiper 15A fuse which is ignition switched. The dashcam now turns on with the ignition.
On the whole this was a lot more work than I expected. If anyone knows a better way I'd like to hear it.