If you go to a decent builders merchants, you should be able to buy 10mm limestone in 25 kg bags, also grit sand in the same size, plus a bag of cement. you can then make your own, which will be much better for your job. Don't forget the feb mix.
I'd take the blinds down and glue 25 mm of pir insulation to the underside of the conservatory with expanding foam, then glue the hollow soffit glued to that.