I'm in Canada, Southern Ontario so we get some cold days and some hot
(-20 to +40 degrees C at extremes but only a few days like that each
year. Winter I'd says a usual cold day would be nearer -10C and summers
in the +30C quite often. I have r20 in basement, and must be close to
r50 attic. the windows are vinyl with low-e double glazing.

Yes it really is a single layer of concrete blocks, each block has 3
cavities in it but I think it would be nearly impossible to fill those
holes with insulation, and anyway there is so much webbing in the block
there would be a thermal bridge of concrete to transmit the cold.

The block is a bit like the 'Split Face Scored' 16*8*8 on this page
except with three cavities
http://www.consumersconcrete.com/pro...splitface.html

Then inside is the 1" strapping (true 1" gap as it's 50 year old wood)
then layer of felt and then the layer os some sort of fibreboard about
half inch thick and a layer of plaster.

Most of the winter it's ok but we get some condensation on inside of
walls on the very cold days, and the fact when we get a windy day the
side of the house receiving the winter wind stays cold. Would 1" foam
in the gap be enough to make the walls warm enough not to condense?
would filling the air gap cause problems - e.g trapping moisture and
actually make things worse by making somewhere for mold to grow and
stopping the wind drying the inside of the wall.