@jkandasa - Gateway reconnected automatically.
It all went bad today - I tried a new 5v pro mini GW (with level shifter for data lines) and that failed miserably. So I made a new 3.3V pro mini GW and a new 3.3v regulator board, and have spent the last 7 hours trying to get things working again.
Sadly I think this will require a full reinstall, losing all data from the hot summer and taking more time than I have available to put into it. Just as it was looking hopeful too......
P.S. Hardware (Rpi3). Add a 100n capacitor across the 5VDC input on the back of the pi (points to attach to are PP2 and PP5). It will filter out spikes on the power input to the pi. An 0805 SMD ceramic multilayer cap is what I used and I suggest the same for anyone else - but I take no resoposibility if you break a pi - do it at your own risk and only if competent to do so.