Sill casting - what would you do here?

I would have thought a can of expanding PU foam would be your friend here! Fill the top of the cavity, and the the gap under the frame. Once set, trim with a saw to your required levels, then shutter up your casting and pour away.

Ah yes, that would be perfect for the irregular void at the end of the sill, but against the thermal blocks, I'd rather use a thin sheet material - like

Don't forget a half dowl along the timber base to form a drip grove in the bottom.

Well no I won't but does it really matter if you do forget. An angle grinder with a worn stone grind disk will easily put one in with a simple guide. In the past I've cast them with a section of flex cable clipped to the bottom of the mould.

This is the bit I'm most interested in opinions on...

Affix a DPC in an L shape against the thermalite before you do anything else, any water that goes behind whatever cill you decide to add will run under the sill and away, provided you leave drain holes, TBH it's not imperative as the joint between window frame and cill will surely be sealed with plastic / silicone / beading / whatever.....unless you are thinking about damp 'penetrating' the concrete and into the thermalite?

Thanks Phil, I won't bother then.

Sash cord works well too.