The out of date CRB is irrelevant to the main points of your question, but the usual suspects tend to pick on the bits they do know about, rather than try attempting answers to the critical questions.
It sounds like you attempted to renegotiate your salary with someone who didn't have the authority to agree such matters, and the organisation is attempting to cover its tracks by now saying the role is not permanent. That way the organisation is not taking on a permanent obligation to grade uplift - many organisations have numbers of roles at certain levels fixed for them by more senior (HR, typically) people to prevent such gradual grade drift.
Of course the offer should have been put in writing - indeed I wouldn't have accepted it til it was, but that shouldn't disadvantage you now.
I think you've tried to pull a fast one by trying to renegotiate after the job was accepted - albeit verbally - and I think you will have to back-off the Grade 5 demand.
I don't have a clue what you mean by 'pay protection plan' unless you are referring to protection given to redeployed staff to red-ring at the current salary. I think you are pushing your luck with this one - it needs a decent HR Manager in there to sort it out.