It looks like Volvo stuck with the down flow top tank / bottom tank rad through the complete production run. If you can, running your hand down the fins to check for temperature differences. That may give you a clue as to what is going on. If the radiator fins mid way down the rad have a significant temperature drop chances are you have a blockage in the rad. Try running let to right to confirm the location of the blockage.
Even with a completely blocked radiator, if the thermostat is opening you will eventually get hot coolant into the top tank resulting in a hot top hose and tank. You could have a sludge blockage in the radiator; but, that usually does not happen all of a sudden. The problem gets gradually worse, usually showing up in hot weather. Plymouth does not look like it has had seriously cold weather. Highs and lows all look to be above freezing. If you have been exposed to below 0C for any significant period of time what is the freezing point for your coolant solution? A pile of slush in the rad will do an excellent job of blocking flow.
How is your coolant pump? Is the drive belt slipping / gone missing / pump seized?
The top tank being hot suggests the thermostat is opening; but, the correct Volvo thermostats have a jiggle valve and even if the thermostat has failed shut the jiggle valve will eventually pass enough hot coolant to heat the top tank; but, that will take a while.
What was your dash temperature gauge doing while all of this was happening?
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