Just before i pursue the passive slab idea further i wondered if anybody here has experience of a traditional slab but using countermeasures for the thermal bridging (floor to wall) such as perinsul or similar ?

 

 

Yes, I did traditional foundations but with modifications to make them insulated. I got the idea from https://www.greenbuildingstore.co.uk/golcar-passivhaus-ground-floor-foundations/

 

My builder altered them slightly In that he has done this before and the DPC above the skirting has lead to cracking so he lowered the damp course to floor level and added a second DPC in the outside skin above the ground level.

Exact same here - using EPS below the DPM and filling the cavity that way is a good method of reducing bridging.