What's the legal procedure for changing speed limits ?

I always thought they needed to advertise etc that it was happening...not just sneak in small speed limit signs and hope nobody notices ?

FFS, they put up huge idiotic signs to tell of a road layout change, yet all over the place here they're reducing speed limits and not drawing any attention to the fact.
There must be literally dozens, if not hundreds of reductions in my area or extensions of lower limits over the past few years.