I'm afraid I abandoned it in the end. It turns out you have to flash the devices with a different firmware. The Sonoff Mini's actually come with a jumper which you have to install inside the device in order to put it into DIY mode and there are a series of instructions which I failed to get through. It was very peculiar because one of the devices was visible on my network but couldn't be seen by the Sonoff software and the other was visible by the software but I couldn't find an IP address for it. I could have been dealing with IP conflicts but two of us locked at the issue and neither of us could work out what was going on.
There is an excellent video on flashing the devices with the Tasmota firmware here YouTube.
If you can work your way through those instructions then you should be on to a winner. As I say, I seemed to be doing fine all the way to the end of the instructions but then they weren't behaving as expected.
I gave up in the end and I purchased some Aeotec Z-Wave relays which are brilliant but much more expensive.