A 25kg block of No.1 invert sugar has arrived from Ragus, and I can see why it's not found in home brew shops. It really is one solid block that looks like it was poured into the bag in the box then left to set solid. It would be a right pain getting it into home brew sized portions.
Its figures for those that care about such things are:
Total sugars: 78.5 - 84.5%
Mineral matter: 0.5% max
Organic non-sugar: 1% max
Moisture: 16.7 - 17% max
Refractometer BRIX: 81.4-81.7
pH: 5.0 - 6.0
Colour: 25 - 35 EBC
Brewers extact: 321.5 -326.5 litre degrees per Kg (also given as 72 - 73 Lbs/2cwt for any imperialists out there).
If I remember rightly granulated sugar is 380 LDK and a moisture content close to zero. If you take into account the moisture content of the brewers sugar its extract is very similar, so not a lot of unfermentable material in No. 1 sugar. I wonder how the darker brewers sugars compare? As ever further research is called for.