Casting concrete sill with no sub sill.
 

I have come across a bay window of a customer, who has sash windows
with no stone sub sill (see link below) instead they have a wood sill,
which has rotted and been covered over with plywood.
I dont know if there was a sub sill originally.
My prefered method to repair would be to cast a new sill all round in
concrete, but my concerns are if I use a DPM between existing wall,
sash frame and new sill. It might not hold in place with the weight of
the concrete (as their might be no grip).
If I dont have a DPM between sash and sill water might soak into the
frame and cause further rot / leaks.
Any suggestions would be appreciated from persons who know what they
are talking about and stick to the point.
Thanks
Martin