Solved it thanks by googling extensively!
The problem was the _Almond extension on the end of my PlusNet wi-fi address which appeared on the Almond screen when I configured it. Once I deleted this, it worked fine. It actually wasn't visible without tapping it to see it in full but, even if it had been, I wouldn't have had a clue! Can't find anything relating to this in the rather brief "instructions", which is more than a little annoying as I spent so much time trying to resolve it (and it only took a few seconds to put it right).
Anyway, now it's working, I'm very happy with it so far and have the full 5 bar signal downstairs.