Depends what you mean by 'legal and still viable'.
The way these things get tested is through an Employment Tribunal. The only way it is going to get tested is if you are dismissed as the result of a situation that uses this FWW (say if you commit another breach of conduct within the period of time the original is in force - typically 12 months) or if you are disadvantaged in some other way - say you got no salary increase this year when others did and the stated reason was that you are subject to a FWW.
In either situation, you would have to take the employer to ET and claim the FWW was flawed.
From what you say (no written confirmation and no given opportunity to appeal) I agree with you that the process is liable to be adjudged flawed. But you are probably best just keeping your head down and working through it until 12 months have past, then trying to antagonise the employer.