Is it a LL mousse? Or are you using CD mix-a-mousse with LL packs?
When I make mousse, I use a bit more water than stated (but use whatever suits you - or what's recommended on the pack).
I use VERY cold water out of the fridge.
Put the shake powder into my blender cup / container and then plonk a heaped scoop of MAM right in the middle so it's 'floating' on a raft of shake powder.
Then I quickly zizz it up with my stick blender (or even better, my Hinari genie blender ... )
You have to do this quickly so the MAM doesn't stick to the sides of the container or begin to clog as it meets the liquid.
After a good zizz, I pour it into a bowl then leave it in the fridge to set (about 15 mins).
Less water gives a firmer consistency ... a bit more water makes it more like blancmange.
Mix-a-mousse is a Cambridge product (I also use it to make jelly out of water and CD flavouring).
I think you can buy it from the Cambridge website (although I get mine from my CDC). Don't know if it would be 'officially' sanctioned by LL but as we use it on Cambridge and they work on the same VLCD principle I can't see why not.