I installed an ubuntu server a week ago with the root and swap encrypted with LUKs.
2 x 1 TB with raid1, LVM and two LUKs partitions.

It did an update today and after a reboot in I get the message 'no key available with this passphrase' depsite putting the correct password in. after many failed attemps I managed to type and see that it was showing the correct password (correct UK layout)

This happens in recovery mode too.

What can I do to fault find?

Thankfully I have a recent backup however I'm wary to reinstall if LUKs is buggy os some other issue
is the cause (defective server courupting data).

I have LUKs on a centos system I installed 18 months ago and my own ubuntu server and have had zero issues. My ubuntu server has the same version and updates as the system that won't accept the password.

Sometimes keyboards are going bananas in strange ways over time, try replacing the keyboard and see if it helps.

+1 to check for a flaky keyboard.

That said, what are you hoping to accomplish by encrypting root (which I’m reading as /, not /root) and swap on a server?
I ask because, as I understand it, encryptIon provides no protection on a running system, which a server would (hopefully) be doing all the time. I thought the purpose of encryption is to prevent access to a stolen or otherwise physically compromised ‘puter.
(If the OP or mods want, we can move that discussion to a new thread...)