I have paraphrased (practically copied and pasted) large chunks of my thesis across chapters. It this worthy of failure or some terrible consequences?

You mean, saying the same thing in a few different ways in each chapter?

Yes.... For instance each of my empirical chapters is essentially a paper I wrote up (not all published) that I converted, and then for the last chapter (discussion & conclusions) I attempted to paraphrase... but errr it may seem quite copy and pastey in areas. I wonder if this will have me sent to hell?

I imagine this happens in almost all theses. My methods for my first two data chapters will be exactly the same, and so the methods sections will be the same too! And the introductions to the chapters will all have to use a lot of the same references and ideas - I'm struggling to make these sound different! But I think it must be a problem all people have, unless all their data chapters are very different from each other which is rare I think!

And after all the conclusion must be expected to paraphrase what has already been said, so it would be odd if the same terms did not come up. Perhaps further rewording would be better though- c&p might look lazy.

Hooray! This cheers me up. I don't mind seeming a bit lazy, I just don't want to be sent to hell.

I thought I'd never finish the thing. I'm slightly shocked and confused that I did.

I suggest you to use https://grammica.com/paraphrasing-tool