Whirlybird, and anyone else interested.
Here's what I do in the 300.
The first thing is to decide at what power level the collective has got stuck at. Are you at hover power or cruise power or somewhere in the middle?
I'll start off with cruise power. If you bring the cyclic back to lose speed then obviously you are going to climb initially. If gaining altitude is no problem and there's no airspace above you then go for it.
If you cannot climb from your present altitude then by increasing your speed you will descend. Then once you are lower then bring the speed back to zero. Once you have lost all the airspeed you will start to descend. It may be necessary to reduce throttle as well. The 300 will fly quite happily with the engine rpm below the green. Obviously be very careful where the rotor rpm is.
If you are descending vertically then also be very careful of vortex ring. It can get quite interesting sometimes. As you approach the landing area then control the descent rate with engine rpm and by picking up a bit of speed again. Once close to the ground I control the descent purely with engine rpm and then do a low speed running landing or a take it straight to the ground.
At hover power I lower the rpm out of the green and you will get the same result. Remember not to be in a hurry as it can take a while to start descending. At the slightest hint of trouble abandon what you are doing.
If I am sitting in the left hand seat (300C) then I will put the friction on the collective, but obviously keep my hand on it at all times. It takes a bit of practice to get it right, but eventually you can land it no trouble at all.