Well you might as well have a go:
Dear London Borough of Hackney,
We challenge liability for PCN QZ1484700A on the grounds that the alleged contravention did not occur. We note there are no advance warning signs on Cazenove Road so our driver would have had no notice of the restriction, by the time the signage would have been in view the driver would already be committed to the turn. We therefore contend that the signage is inadequate.
Yours faithfully,
I fully expect a rejection, post it up on here when you get it.