The sign is quite new, but it's almost right on the turn, with restriction times that have to be read whilst one is on approach, so in my view this is inadequate under Regulation 18 of The Local Authorities’ Traffic Orders (Procedure) (England and Wales) Regulations 1996 : -
https://www.legislation.gov.uk/uksi/1996/2489/regulation/18A timed restriction like this needs an advance sign so that a motorist can, at the 20mph limit, read the restricted times, and not at the point of turning. However, whilst this is a valid argument, it would only get tested at London Tribunals, the council will inevitably refuse any representations you put forward, and hide behind the fact that the discount option ceases at the Notice to Owner stage.
There may also be a 'technical' appeal based on B & D mismanagement of the enforcement process, so wait a bit to see what may be suggested, but don't miss the deadline to pay or submit representations.