I have an old iPad registered on an old (defunct) email address. Cannot switch off “Find my iPad” and hence can’t register to new AppleID…..help!!

See if you can reset your password. If you forgot your Apple ID password - Apple Support

As far as the iPad is concerned, the email address only has to match what the iPad has stored as the address and password. If you truly just "changed" your email address on your current account then the iPad should know that if it can connect to the Internet. If by "change" you meant you created a different Apple Account then the iPad will insist upon the information from the old account. In that case your only recourse may be to ask Apple if they will reset the iPad. Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not state what currently constitutes proof. Prior to this new online service Apple typically required the original receipt issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).

Apple recommends you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support