Thank you @HC Andersen and mickR. So this is how I am going to challenge it, which is what you wrote with the extra remark about the turned sign from mickR'
Dear Sir/ Madam
Ref PCN xyz
I wish to challenge PCN [number] on the grounds that the alleged contravention did not occur. (Do I add this part?)
As can be seen, I parked in a parking place opposite 1-6 Tensing House. This is approx. 30m in length and is not sub-divided into individual spaces.
I was aware of the regular parking restrictions and from where I was parked I could see this large sign over the roofs of the cars parked ahead. It was uncovered and I had no reason to investigate further. I did not see the suspension sign which hung under the regular sign and which was obscured by car roofs, and anyway it appeared to be angled away from the suspended section.
When I found the PCN I investigated further and it was at this time that I saw the suspension sign which described the suspended area as follows:
'2 parking spaces(10 metres) OPPOSITE 1-6 Tensing House'.
No attempt was made to include a diagram or map which would have been a relatively simple matter and I believe is common practice.
Presumably the council thinks that this description had a unique meaning of the first 10 metres (measured from the furthest point from the sign itself and nearest Tensing House) of a 30-metre parking place which does not contain any 'spaces'. In my opinion the wording is ambiguous and I should not be penalised for parking in these circumstances.
I think your argument is as I drafted (I would, wouldn't I) plus mickR's point.
With respect, you're falling into the trap of now knowing what the sign was supposed to convey and then looking at the sign and circumstances. This is wrong. By way of analogy, some time ago I wrote some spoof regs about erecting direction signs, the main thrust of which was that these should never be located by anyone who knows where the places referred to are. It might seem odd, but it's not. Only people who don't know where somewhere is can decide how many signs are needed and where because anyone who does know cannot possibly see the issue from the perspective of someone who doesn't, for whom of course the signs are intended.
What would be the UNIQUE meaning of their suspension sign? IMO, there cannot be one, it's ambiguous to an uninformed person and therefore fails the legal test.