We do see rather a lot of these PCNs where the restriction is at the street on the right on coming up to a T-junction.
To the council's credit, they have put up an advance sign of the type we like to see, of a plan of the street layout ahead, and where one can turn or not turn. You say it is obscured by the trees, but that certainly doesn't come across in your photos. The sun glare is more valid, but this is a problem anywhere, not just here. What could be criticised is, there is only one actual 'Flying Motorbike" sign; there really should be two one each side of the street entrance.
Councils have a duty under Regulation 18 of the The Local Authorities’ Traffic Orders (Procedure) (England and Wales) Regulations 1996 to erect 'adequate' signage. However, like all weasel words like this, it is subjective. To you and your wife it is inadequate, to the council it is adequate. So they will refuse your representations leaving you with the choice of paying the discounted penalty, or registering an appeal at London Tribunal with the full PCN penalty in play. The adjudicator would read yous submission and the council's and decide if the signage is adequate or not.
So the bottom line is - are you prepared to stand your ground and take them to London Tribunals ? For me, you're not on strong ground as there is an advance warning sign that doesn't appear to be obscured, (looking at your photos). However, you have the absolute right in law to put the matter in front of an adjudicator, but you must first submit representations to the council and have those rejected, before you can do so.
