If it was a 'no stopping' area (rather than a 'no waiting' area) it should have been marked with a single 'extra-thick' yellow line. (The only exception would be a 'red zone', where the red lines do the same job over a longer length of road). The 'no-stopping' regulation will always apply for 24 hours, unless a plate indicates otherwise.
Such regulations tend to apply at, or around, bus stops. (Although, of course, they can be marked anywhere where stopping might impede the flow of traffic). If you stop, for example, to drop a passenger off, it's your duty to check that there are no 'no-stopping' markings (and, if you can't read find or read the plate, to assume that they apply at all times).
As stated, the only effect of a plate on a lamp-post (or elsewhere) is to indicate the times when the regulation does not apply. If there's no plate (or it's positioned where you can't see it) the extra-thick yellow line (or the red line) obliges you to assume that stopping at that point (even for a second or two) is an offence.
Chris