But looking again at the hydrometer, it should have been 1.034

Nevertheless.... Oh dear.

Possible explanations
1. I somehow cocked up the hydrometer reading even further - bubbles, miniscus etc...
2. The concentrate wasnt fully dissolved, some stuck to the bottom
At least the yeast seems to have taken, the top half of the airlock was fired across the kitchen floor! Steep learning curve here guys.....
edit - Looking at the temperature adjustments, it seems the OG is actually 1038ish depending on if its calibrated to 15C or 16C, which is a bit closer to the mark. Hopefully the rest can be explained by inexperience with using hydrometers!