I have tried the Spellcheck option as suggested by Tex2002ans and I get the following result -
https://www.mobileread.com/forums/at...1&d=1610198353
The language has defaulted to US English (the book was published in the US) so it is now picking up words that I have already corrected to UK spelling as being misspelled.
I have tried clicking on the Language column to see if I can change it to UK English so that it spellchecks against the default UK dictionary, but there appears to no way to do that.
What am I missing?