Thought I’d share this for others now that it’s working, maybe save you some time … cos every little helps.
Gear
1x Pi3B running HA with Bluetooth [BLE Monitor, HA deConz integration]
1x Pi3B running Raspian with Raspbee II [deConz/Phoscon Pi app]
GOAL #1 : Does the plant need water?
Install HA & Mi Flora integration on a PI3B. Everything immediately connected to the Xiaomi Bluetooth Low Energy (BLE) plant sensors 
Ditched the Mi Flora integration and installed the BLE Monitor (apparently better on battery). Worked perfectly no problems.
Ordered a bunch of rip-off xiomi sensors from China for literally half the price (£12/$15) they connected first time. [get the white, international ones, not the green ones]
GOAL #2 : Oooh the Zigbee.
Next plugged in the Dresden Elektronik Raspbee II to the HA Pi.
Nope.
Fiddled and faffed. Nope.
Phoscon, Dresden, deConz just nope.
Discovered PI does not support Bluetooth & Raspbee II. (get the Conbee USB stick rather)
Read that if you edited the config.txt you could disable BT so Zigbee would work. Discovered editing the config.txt is a DRAMA because HA runs in a docker container and so can’t the host partition where the Pi config.txt resides. Decided to ditch this approach cos I would still need BT.
Soooo… dusted off another old PI. Plugged in the Raspbee II and imaged an SD card using the Dresden image.
Booted up into deConz, and saw the RaspBee II first time. yay.
Clicked on “Phoscon App” and stepped through adding Zigbee devices.
- IKEA Switch E1743 = worked, but omg what a pain. try and try again. it will work. Make sure you get a red light,
- IKEA lightbulb = worked first time.
- Linkind sensor = worked first time.
I’ll update the other equip as I go along…
So now I have HA on one PI (HAPI) and deConz/Phoscon on another (oohTheRaspbee)
Both work individually. Guess it’s time to install MQTT to connect them.
Head over to HAPI (the one with BLE) and it says it found OohTheRaspbee [I guess the deConz integration was still installed from previous mucking about ]
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But its telling me to: Go to deCONZ Settings → Gateway → Advanced
Press “Authenticate app” button
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What is it on about??
Turns out that in addition to Phoscon App there is another app, summoned from 2016 by clicking ‘WebApp’ button in deConz. This opens a portal to the predecessor of Phoscan as far as I can tell, but before you can enter you must tell it that you are a delight. Seriously you are a delight and your password is delight.
So after telling a WebApp from 2016 that I’m a delight I’m into a whole new old world, but still don’t see anything that matches the instructions to click ‘autheticate app’ so I try “Unlock gateway”
ka-chiiing!!
Suddenly HAPI is speaking to oohTheRaspbee and zigbee things appear and work just fine in HA.
Well what a pleasant surprise!
And as I’m typing this Alexa announced she’s devine too.