There's three types of heater control, but in essence they're all the same...
they are held in with two Phillips style scews, one at the top and one at the bottom. They are located and accessed only from the rear of the contorller.
It maybe easier just to use a mirror and a torch and put the tube back on...
Failing that, you could pull off the rubber plug that connects the vacum tubes to the heater valve, this would give a little more acess to put the tube on the rubber plug.
Does the tube that's dangling have a screw in it? and is it coming of the back of the rubber mounting plug?
TTFN
If you have a problem, if no one else can help, and if you can find them, maybe you can hire... The N/A-Team.