I thought that the 14-day grace period for paying the discount rate began when the PCN was served.
It depends on the legislation, under this particular Act of Parliament the discount period is 14 days from the date of the notice.
At this point the representation is only there to buy us some time, so it hardly matters what you say as long as you avoid anything self-incriminating.
Something like this should suffice:
Dear London Borough of Lambeth,
I challenge liability on the basis that the signage does not comply with regulation 18(1) of The Local Authorities' Traffic Orders (Procedure) (England and Wales) Regulations 1996.
It follows that the penalty charge must be cancelled.
Yours faithfully,
Hopefully by the time you get the notice of rejection, we will have the traffic order. The notice of rejection should reoffer the discount for a further 14 days from the date of service of that notice, so we'll have a bit of time to consider next steps.