Endymion Ponsonby-Withermoor III ) gurgled happily, sounding much
like they were saying :

Following from the Tower Bridge thread, can anybody explain to me the
logic that "unsigned roads are subject to a 30mph limit" ?

I sort of remember this from when I took my driving test some 20+
years ago but thought it was stupid (and ambiguous). However, I keep
reading reports of people have been prosecuted for allegedly going too
fast where no speed limit was posted.

How can this be ? As far as I am concerned, whenever I see a speed
limit sign facing me, that speed limit is in force until I see another
one (lower or higher than it), right ?
(Obviously, some signs get vandalised and obscured)

but what on earth constitutes an "unsigned road" ? How far does one
have to travel on a road without seeing a sign, before declaring it
"unsigned" ? I seem to remember that this "default 30mph limit" only
applies (a) in urban areas, (b) where the street lamps are 185m
apart.

If I see a 50mph sign, travel for several km at say 47mph, and
an urban area (discuss definition) shows up, does that mean I have
to slow to 30mph. How do I know when I can speed up again ?

This seems a very poor definition.

Is it something like the NSL marker where, (for an ordinary car)
wherein the speed limit automatically changes 60/70 as the road becomes
single/dual carriageway ?

Repeater signs.
From the bottom of the speed limit table in HC103 - "The 30 mph limit
applies to all traffic on all roads in England and Wales (only Class C and
unclassified roads in Scotland) with street lighting unless signs show
otherwise)"

Basically, all speed limits except the default must have repeaters every so
often. If they aren't there, then the default applies, regardless of the
last "big" sign. That's 30 if there's streetlights, NSL (60 S/C, 70 D/C) if
there aren't. Motorways are default 70, streetlights or not.