Thank you for your replies. Yes, I have been using Calibre's inbuilt editor and find it to be very helpful. Being a relative newbie to html code, I find the 'find & replace' within the editor to be much easier for me to use than the 'regular expressions'. I had hoped there was a proofreading method within this editor.
I just find it takes away the enjoyment of reading, from a book with numerous errors. These are all purchased ebooks that the authors and/or publishers should have proofread before sale. I shouldn't have to do it.
Thank you for alerting me to Sigil. Will give it a go. Is there [basically] a
dummies guide to using it, though?