Update:
Some things have changed since yesterday; I had a power outage so the Raspberry PI which Home Assistant resides on ended up being restarted.
And now I have just managed to work through the steps and the integration seems to work (I think anyway and I had a Success! message and I can see the Smartthings tile so hopefully all good.
Perhaps a restart was all that was needed? I should have tried that earlier…
In any case, thank you all very very much for all the help you have given me.
Now for the Google Assistant integration…