On 19 Dec, 18:56, John Rumm wrote:

Insulating blocks could refer to any number of lightweight concrete
blocks used for the inner leaf - it does not mean the cavity will have
any other filling necessarily.

Thanks. I didn't know that's what that could mean. I thought it
referred to actual insulation within the cavity, such as I have seen
being used in houses being built recently (looks like some kind of
expanded foam with a foil backing.)