Hello there @timjeffries 
First of all, let me say that I’m truly sorry to hear about the issues you’ve been encountering. What @flowmondo is saying is correct. We have been experiencing authentication issues with Xero and we’re doing all we can to resolve them.
For more info, you can follow this community thread: Failed to verify connection 'My Xero connection'. Status Code Error: 400.
Sorry again for the trouble this has caused and thank you very much for bearing with us 