We've seen this location before, and the OP was successful on the grounds of inadequate signage, but it may have been enhanced since then.
The traffic lights are a sign just like all other signs, and here are wrong, because the permanently displayed Left Turn arrow is a mandatory instruction, yet the Buses Only sign beyond it is day and time restricted, so the signage here is a complete mess, frankly. There should be an advance sign with a geographic map of the junction showing the restriction ahead and its applicabble days and times, and the traffic lights should be changed to reflect the Buses Only days and times.
Of course if you decide to register an appeal at London Tribunals, it will be with the full PCN penalty in play, so clearly it is your decision and a win is not guranteed the same as in any other court of law, but I'd say your case is reasonably strong. You could try searching the London Tribunals Statutory Register for the location on the PCN to see if there are any previous cases here and what the result was.