My eldest son came to visit today, so I took the opportunity to get him to help fitting the trusses. I'd thought that they'd obstruct the pillar tops thus preventing me filling them with concrete, but in practice they don't. Construction is rather unconventional, in that I've laid a 'ceiling' of 20 mm shuttering ply from wall plate to wall plate, then set the trusses on top of the ply, as fixing a ceiling after the trusses are in would be very difficult in such a cramped place.
It was while measuring up to cut the ply we realised that Brian the Brickie had set the lintel in the wrong place - it was 4 1/2" too far forwards. So I chopped it out, and there is still access to re-set it even with ceiling and trusses in place. He's back this evening to finish the pillars, so he can re-set it then.