Now, I found this out quite by accident.
I was changing my batteries on my cordless keyboard and I must have pressed some random keys because when it started working again my ‘@’ button was giving me speech marks and Shift+3 produced a ‘£’ instead of the ‘#’ symbol. I was most confused.
I guessed it was the keyboard language that had somehow switched to UK, rather than US English, as I had both set up. So how did I change it back?
Press the left Alt key and Shift.