Enter Passcode for Other iPhone does not recognise my Other iPhone password

I am setting up my new iPhone SE and I have loaded my Apps and Apple ID successfully, but to transfer the keychain and passwords it has asked me for my Apple ID password, then the new phone unlock passcode, after which it says "Enter Passcode for Other iPhone". When I do it says the passcode for my other phone is wrong, even though the passcode successfully unlocks the old phone. Any ideas, as it says the only other way forward will erase all the encrypted data from the new phone. It does say I can authorise the device from another device, but I just get the circular timer going round and seem stuck... Thank you!