I'm currently finalising my corrections (new chapter and new conclusions) and realising how much more I have to do before I submit the revised thesis next week. My worry is that the new stuff will be so rough around the edges (i.e. too long and rambling) that the examiners will not accept them. I can't find any information in my university guidelines about what happens next if the examiners are unsatisfied with the corrections. Will I get yet more time to make more changes, or will that be it and will I fail?

I don't think you need to worry too much. The examiners should already have made the decision about the outcome immediately after the viva (e.g. pass with minor corrections). And your job is to revise the thesis to the satisfaction of the examiners. So if they are not entirely satisfied with the revision, they will ask for further changes. It's unlikely that they would go back and change the outcome.

At the very worst you could get major corrections, but If you have followed the instructions of the examiners you should be fine.

Thanks everyone. I just am writing so slowly that I don't know what the quality of the work I submit will be, and I'm scared the examiners will hate it and say it's not good enough. I just want this process to end.