Bitcoin Core Wallet doesn't accept my passphrase I am using Bitcoin Core Wallet .21 and have encrypted my wallet.
When I try to send some coins it asks for the passphrase, which it says is incorrect.
I have only ever used one of two passphrases and I have copied and pasted these each time using notepad (upper and lower cases), but it rejects my passphrase every time. There is nothing wrong with my passphrase, I know my passphrase is correct.
Are there any circumstances under which the wallet would behave like this?
I have used old backup files of my wallet, but get the same message.
Could my wallet be corrupt and if so can it be repaired anyway?
Are there any circumstances under which the wallet would behave like this?

If your passphrase contains non-ASCII characters, then you could have an encoding issue. I would try typing in the pass-phrase rather than cutting it out of Notepad and pasting it.
Unicode has multiple normalisation forms and this is another thing that could theoretically trip up software.

Could my wallet be corrupt ...

Yes, but your backups show the same result. If you successfully used the passphrase anytime after making a backup it is very unlikely that wallet corruption is the cause.

... and if so can it be repaired anyway?

No-one can tell you that. Repairing an encrypted file is improbable.