I work in a room in my house. ,  always warm and.cosy

Yeh there's quite a few epoxies out there that state they are non-blushing. 

Ah, see small amounts are very difficult to measure accurately enough. With epoxy you should always be within 1% accuracy. For 12g, that means you can only be 0.12g out. Are you confident you were within that margin? 

I am using digital scales and mixing up 12g at a time (9g resin, 3g hardener).

I would also tighten up on your weighing and mixing as slight errors in these can all contribute to the problems you are having.  When the mix ratio or mixing itself is out, the cured hardener components can come to the surface as with blushing causing the same problems.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Just at home, I don't have access to workshop conditions.

Just took a look at the TDS, and it appears the XCR is in fact very sensitive to amine blush. 

Amine blush. It's a reaction between the amines in the hardener and moisture/carbon dioxide in the air. There's various types of amines, and some of them are much more prone to blushing than others, which is why some epoxy products may be deemed 'non-blushing' - they use an amine product that is much less sensitive (or in some cases, don't use any amines at all). 

What's blushing?