Difficult one, this. You drove over the junction head-on to the sign, although it has to be said two signs really should be present, but I think an adjudicator would conclude the sign is clear enough. Against this, is the fact that a driver's attention has to be elsewhere when making a move across a junction, so would be looking out for approaching traffic from right or left.
So it's probably best to submit a non-confrontational representation stating you are a private hire driver on the way to pick up a passenger. Explain that you are very careful to look out for signs such as the one you passed, but taking care at the junction to ensure it was safe to drive across, you seem to have missed the sign, of which there is only one. State also that you realise from the PCN that you were only 5 minutes inside the restricted period and ask that they cancel the PCN on a goodwill basis, due to the minor nature of the offence.
