Just the two bedrooms to go now! The slightly darker patch of ground is what we dug last night in 2 and a bit hours. The hole was what looked like rotting timber, which would decompose further over time and lead to a sunken patch of floor - best to dig this out and fill with hardcore. We've found a few broken bricks and things so I've thrown them in there to get us started.
NHBC inspector confirms the next two inspections can be done at the same visit, so we're looking at the week from the 6th to the 10th July to finish the walls, lay and compact the hardcore and lay the dpm, and get all that inspected - ready for pouring the ground floor slab the following week. So a provisional date for expected completion of 'Stage 1' is the 26th of July. Which is exactly what I have on my schedule (after adjusting it an extra week for my gammy arm).