Broken slate is alright for sub tbh. If you're still at 260mm, you can just chuck a couple of bags of mot down as is. I wouldn't want to blind off over slate as it may still puncture the dpm, you've probably just about stopped digging in the right place.
If you've a whacker plate then this is on the way to being a top notch job. You getting concrete delivered?
Yes mate, the original floor was approx 200years old Caithness slate flagstones, with whats known locally as 'sea scrapings'(sand and shells) as a sub base, as we've been digging this out we have hit loads of slate pieces (and bones!!) that must have been thrown in to give a more substantial base.
The base is still quite sandy although stable enough (200 years in the making).
What do you think??