ZigBee mesh problems - no repeating

I have more Z-Wave devices than ZigBee but I am now trying to create a house wide network for ZigBee, but having problems. Devices local to the hub seem OK but I’m unconvinced that the mesh is working over multiple hops created by using SmartThings UK Outlets, of which I have seven.

First to clarify I am always adding these to ST by pairing them quite close to the hub. That is right isn’t it - network wide inclusion isn’t appropriate for ZigBee ? I then move them further away to the edges of my network . I have for example one ST outlet that works reliably and I have placed a new outlet just 5m further away in direct line of sight of the working outlet. This new one doesn’t then work. If I bring it closer to the hub it does work. I have left it for several days with no change.

I have also tried removing power (and batteries) from the ST hub for well over 15 mins, indeed even overnight and then re powering to cause a ZigBee network heal. Again the distant outlet(s) have not become controllable even after a couple of days.

I could only think that some RF issue was interfering. I do have two other ZigBee networks , Hue and one other, so I tried to add the outlet at another extreme of my network - again within 3m line of sight of a working outlet to no avail. I also tried turning off all WiFi but it is still heavily congested here. I have tried toggling power to existing outlets but I just can’t get any distant outlet to be controllable (one that is requiring an intermediary ST controllable Outlet repeater). What more can I try ? I am toying with getting the XBee bits to see the routing map.

I have one further question - do the ST Presence sensors work via repeaters or do they only work on a direct to hub connection ? I assume the motion and multi sensors do work via repeaters.