Here is a simple draft:
Dear London Borough of Waltham Forest,
Having now taken legal advice, I challenge liability on the basis that the authority has defaulted on its duties under regulation 18(1)(b) of The Local Authorities' Traffic Orders (Procedure) (England and Wales) Regulations 1996: there used to be three regulatory signs for this bay, but the one that would have been right behind my car has been removed, one was obscured by vegetation, and the only one that would have been visible was out of sight at the end of the road.
As there was no signage visible from where I parked my car, the alleged contravention did not occur. I would also point out that if the authority saw fit to provide three signs in the first place, it is under a legal obligation to maintain them.
In light of the above the alleged contravention did not occur.
Yours faithfully,
Send the representation online and keep a screenshot of the confirmation page. If the council issues a rejection, I'll sort out the tribunal appeal for you.