Do I need to obtain consent from a member of staff if we have taken the decision to restrict/alter their duties?

If the duties are so fundamentally different from their contracted role, then yes. For example, if you are asking a frontline clinical member of staff to undertake administrative tasks in another area, then this will be a fundamental change to their terms and conditions for which you need their consent.

If there is a minor alteration to their duties, or the clause within their contract is wide enough to cover their amended duties, then arguably to do not need their consent but best practice would be to obtain their agreement.