Maybe a fault on the sound carrier channel at a certain transmitter... If it had been a fault at C4's end it would have affected a larger area than just "the South" as C4 is split into six advertising regions (known as LEMNUS) - all of which are controlled from their London HQ:
L
ondon
E
ast/South
M
idlands
N
orth
U
lster
S
cotland
Ah, is that what LEMNUS stands for. I'm sure the word used to appear on their PDC engineering text pages, before they wiped those off when 4-Tel was abolished.