After around 18 months of planning things have finally been moving a lot faster over the last couple of weeks which is great! I'll try to summarise the interesting bits:
Type 2 vs. Type 3
Our EPS was installed on a sub-base of 150mm MOT Type 2 and 50mm sharp sand for blinding. Structurally this is absolutely fine, but there was an awful lot of and fro with the foundation designers about if permeable type 3 + grit should actually have been used as specified in the system certificates. I believe the only potential down-side of this approach is the potential for capillary action causing water uptake in the EPS and reduced thermal performance, but, given we upgraded to 300/400mm EPS this shouldn't be a significant concern.
Drainage below EPS
While it was a challenge with the invert levels (especially with the 400mm EPS) we decided to try and ensure that all drainage went through the sub-base and not through the EPS. This resulted in a redesigning the drainage runs as well as ensuring we use inspection chambers without drops, but it all works in the end, with just small notches being required in the underside of the bottom 100mm of EPS in a couple of isolated locations.
"The bend at the foot of the stack should have as large a radius as possible and at least 200mm at the centre line"
One warning when putting drainage in. Make sure the correct shallow bends are used! Our building inspector had, in theory, given us a pre-pour approval, but then when he got the drainage photos it was clear that the correct bends hadn't been used for first floor SVP's which he flagged up, so we had no choice but to mine through 300-400mm EPS and change them!
Why so much EPS?
Given our house design has stepped sides (something to do with street scene according to our architect) this means there are numerous steels and load-bearing walls internally. What this meant in practice for the foundation design was that around 60% of the slab needed reinforcing and would have 250mm concrete and only 200mm EPS. We had a u-value calculation done based on our actual foundation design and as we expected the u-value wasn't great, so we decided to go ahead and upgrade the EPS to 400mm which ensure there is a minimum of 300mm EPS across the while slab. It might have been overkill, but the price to upgrade wasn't that much and our PHPP calculation was already assuming 0.10.