Your joint savings are in an account of his name only which you do not have access to?
The joint income from renting a joint property also goes into another account that is only in his name?
He's now saying that he will let you have access to joint savings and the joint income that's half yours but only if you transfer the £1,000 you were underpaid in benefits?
No, I don't think that's ok. Fair enough if your savings really were joint and you had equal access and equal say in how they are used. if you needed money from the "joint" accounts do you have to ask and explain why to him? If the answer is yes, does he ask you if it's ok and tell you why when he's taking cash out of them himself?
Why can't he just let you know the passwords now? Assuming his no writing passwords down rule apply to himself, they are just in his head and he can give them to right now, he doesn't need to arrange to give you them.
Joint savings are fine, except yours isn't joint, he has it all in his personal account. I think it makes sense for him to out half of the joint rental income and half of what's in the joint savings into your account, you can access it anytime and won't need to worry about passwords as you'll only need one. (There's many brilliant apps that store multiple passwords safely)
Make sure you see statements so you know it actually is half too.