OP, unfortunately 'substantially compliant' is subjective and there will be views. But for you only one view counts, and you cannot ask that person here.
For your benefit, I will try and break down the issue.
IMO, the matter wholly turns on whether the council has placed 'on or near the road .. 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;' because if an adjudicator disagrees then the burden on the council of proving the contravention has not been met and your appeal would be allowed.
In this case the 'traffic signs' are DYL and double kerb blips. In both cases, substantial compliance with the Traffic Signs etc. Regs. is the standard. It is also recommended that DKB be placed between 2 and 4m apart, preferably 3m, so that in every circumstance a driver must park alongside one or 2 sets of DKB because there won't be an upright sign to consult.
Here, the authority is focusing on a single set of DKB, and if you know where to look, because it's front and centre in a photo, then arguably it's visible.
But IMO what is unarguable from their perspective is that the DKB are not as visible as the DYL and if one was looking for another marking with equal prominence, then you would be likely to not see the DKB. And given that these markings come in pairs i.e. DYL and DKB then, IMO, in order to ensure that adequate information as to the [combined] effect of the Order is made available' the council should ensure that equal prominence is given to both parts AND that the DKB are spaced as recommended in the Traffic Signs Manual, Chapter 3. I cannot see another DKB in view.
IMO, make reps but flesh out and not just state 'substantial compliance'.