My comments in blue below.
Aominedinburgh wrote:
I don't know how it happened. I didn't set out to encrypt it, I didn't realise it was happening. Could it have been set to encrypt as default? Yes.
I was never offered an opportunity to set a password - if the system had offered the opportunity I would probably have realised what was happening. I tried using my Apple ID password, but it wasn't accepted. The box just said Password. No explanation. It must have been set to encrypt already and I didn't notice. Likely so, see article below.
Please read this article, particularly the later part. It looks to me like you have a new "problem" now as your backups will always be encrypted from now on unless you completely Restore your iPad from scratch, a time consuming process.
About encrypted backups in iTunes - Apple Support
To Restore the iPad:
Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support