Thank you for the replies. I have now checked a few things. I generally use Linux MINT with Firefox browser for most day-to-day things. However I booted into Windows 10 and the same issue does not ocurr there. I checked both Firefox and Chrome browsers. So I booted back into Linux MINT and checked the Firefox language, which is set to English/United Kingdom. I also checked the system language which is set to English(UK).
I have attached a screenshot of the paragraph above in edit mode to illustrate what is happening. I tried unchecking the tick next to 'Check spelling as you type' and this does remove the problem so this issue is evidently related to the browser dictionary as has been suggested. It doesn't seem to matter whether I choose English/United Kingdom or English(En) as the language, I still get the majority of correctly spelled english words underlined, so still baffled.