Contravention did not occur.
IMO, your draft is unnecessarily long and introduces secondary and worse issues which do not bear upon the key point: the number, type, scope, location and method of display of the traffic signs, both regulatory and for information, at the point at which the restriction in question applies - the restriction not being accompanied by any prior warnings on approach - makes a relevant motorist's ability to see, read and act upon the regulatory sign in question impossible. The authority have therefore failed to meet their mandatory obligation under Regulation 18 (Traffic Signs) of The Local Authorities’ Traffic Orders (Procedure) (England and Wales) Regulations 1996 and thereby may not demand a penalty for the alleged contravention.
As regards s121B RTRA, since when would a failure by a council remove a traffic authority's power to demand a penalty from a motorist? And IMO it's not relevant anyway and who's to say that the GLA(not TfL) road has been (adversely) affected?