Thanks PinkElephant. I found my mis-spelt word in all four installed dictionaries on my system.
In Windows 10, they're in the folder %APPDATA%\Roaming\Microsoft\Spelling\
I've accidentally added a misspelt word to the dictionary and I can't work out how to remove it, nor find the location of the dictionary to edit it.
Does anyone have a solution for this?
Many thanks.