What to do with this mess of a concrete sill?

[Kilt]

I’ve just had a new window put in, and was checking underneath for a draft. 

Ive found the concrete-precast sill has no DPC or much of anything behind it.

There was a load of bricks just chucked in the gap, prior to photos. 

The external building is a rubble-stone barn conversion, all done in lime. 

what should I do, back fill the sill with lime mortar, to try and minimise water ingress? 

 

no way of getting a DPC in. 

[Kilt]

[Ferdinand]

If it needs no more structure, I'd fill it with EPS, hopefully from a single offcut shaved to shape - possibly foamed in to fill the gaps.

 

Or polybeads in a mixture with a bit of PVA, so they stick together and don't fly out again.

 

The important thing is to vanish the cold bridge.

 

Or you could make a secret compartment with a lifting lid.