If your heater is stuck on hot (or cold or whatever), then most likely the control cable has become disconnected from the control knob. This can happen a number of ways:
Sometimes the catch on the rear of the knob snaps - in which case you need a new knob or entire heater control panel.
Sometimes the end of the cable snaps where it forms a hook to attach to the control knob - in which case you can usually form a new hook from the end of the cable and get away with a slightly shorter one.
Sometimes the cable simply disconnects itself either from the control-knob itself or one of the retaining clips pops off - in which case you can normally reconnect everything after copious amounts of sweat, tears and colourful language.
The root cause of all the above seems to be that sometimes the control cables are nicely coated internally with PTFE or whatever and slide freely in their sheaths. Othertimes they're not and take quite a bit of effort just to move in their casings which puts too much strain on everything else. I think at this stage I've done four of these jobs, maybe five, and in each case once I lubed the control cables and refitted have ended up with a vastly improved control set.
I currently have a heater unit out of an old one and could take a pic or two if that helps.
Of course, to get to the heater console you first have to move the centre arm-rest back, unscrew the radio console and move it out the way etc. A little bit of bother but quite straight forward really.
As regards the lack of puff; is the fan actually whirring away healthily? Sometimes again, the air-direction control cable misbehaves and stays in 'recirculate' mode, which give the impression of no air. Do you get a good blast of air with no fan but the vents open at, say, sixty miles an hour? If not then the air is being redirected.
Otherwise it's probably the fan-control switch or else the motor itself.