I use my food mixer to whip froth into a cold (but not yet set) jelly, when it is full of bubbles, I then add a muller light and keep whisking until the whole thing about doubles in size and becomes all froth.
When you set this, it is a lovely mousse. A whole pack of sugar free jelly is 2 syns (although 1/2 a packet whips up nice with one muller so using only 1 syn for an enormous bowl .. sort of 4 portions worth!) (or use 2 mullers to a whole jelly sachet - 2 syns)
orange jelly + a choc sprinkles muller
strawberry jelly + strawberry muller
lemon and lime jelly + vanilla muller
you get the idea