I think that deciding to put it either in the first or third person should have been one of the very first things that they should have done!
With regard to making mistakes, it’s clearly not acceptable because that’s the reason that you engage them in the first place!
I don’t know what the book is but of this is a consumer contract then the Consumer Rights Act applies and if it’s a business to business contract then the Supply of Goods and Services Act applies although the provisions of the same:
They have to carry out the job with reasonable care and skill and
If no timescale is agreed, and the timescale must be reasonable.
I think by anybody’s interpretation, between December 2019 and now is not reasonable and in any event, you say they had 280 working days which they would in breach of and as they have taken almost twice that, then to my mind this strikes to the very heart of the contract and you would be entitled to rescind and have return of all your money.
As I say in my first response, this is only too common. That’s why I guessed what had happened from your initial brief post.
Ultimately, if they won’t refund you and you cannot come to a settlement, your only remedy is to take them to the Small Claims Court.
The new Beta system is much easier to use: https://www.gov.uk/make-money-claim than the old system.
You need to give them a deadline, in writing, telling them exactly what you want and that if they don’t do it, you will issue court proceedings against them. Prepare the proceedings now and save them ready to pop in the post when the deadline expires because I can tell you now, from experience, they will not pay up without a fight.
Don’t be at all surprised if once you issue court proceedings, they do everything properly and arrives with you in their defence is that “we have done the job now”. The voice of experience.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have