They are not "double yellow lines" they are "a road marking to diagram 1028.1".
There's a bit of case law saying, basically, that even if a sign looks right that's not good enough, it must be right.
Assuming this is in a Controlled Parking Zone there are more requirements - the CPZ must have signs either side of every entrance to the zone. Every edge of every road within the CPZ must have double/single yellow lines/no loading blips or a marked parking bay. (presumably if any part of the zone doesn't have this the entire zone isn't enforceable?)
If we have to obey their laws, they have to obey their laws too.
Also the PCN and NTO have to be worded correctly or they aren't legally recognised.
And the restriction (CPZ) is actually a document. This document will specify what restrictions apply where, and where the signs should go.
Ask the council for a copy?
TSRGD 2002Single/double yellow lines:
1017 - 1018.1The people on PepiPoo have more information.