Well, they are talking tosh, (as usual !!).
These zones are set up under a Traffic Order, and the council are under a series of duties under the various regulations in the The Local Authorities’ Traffic Orders (Procedure) (England and Wales) Regulations 1996
The key regulation is found in Regulation 18 which relates to traffic signs. These must be placed so as to give adequate notice of the restriction, (see 18 (1)(a) below)
18.—(1) Where an order relating to any road has been made, the order making authority shall take such steps as are necessary to secure—
(a)before the order comes into force, the placing on or near the road of such traffic signs in such positions as the order making authority may consider requisite for securing that adequate information as to the effect of the order is made available to persons using the road; (my bold)
(b)the maintenance of such signs for so long as the order remains in force; and
(c)in a case where the order revokes, amends or alters the application of a previous order, the removal or replacement of existing traffic signs as the authority considers requisite to avoid confusion to road users by signs being left in the wrong positions.
However, they have re-offered the discount. If you decide to take them to London Tribunals, it would be with the full PCN penalty in play. Your representations, like those submitted to the council, are on the basis that the signage is inadequate. However, this is subjective, so you'd have to convince the adjudicator that this is the case, whilst the council will try to say otherwise. The fact they put up an advance sign is in their favour, but the sign not indicating the restriction was beyond a mini-roundabout is against them.
If you win, you pay nothing, if you lost you pay the PCN penalty, but that is all, there are no other costs. The council have to prepare an evidence pack and pay the adjudication fee which they don't like doing, so may not contest, but you won't know until a few days before the adjudication.
So it is your choice, really, risk an extra £80 for an unbiased decision.