The sticking is unlikely to be a problem with the heater unit.
It will be more likely one or all of the below.
1- the mechanism behind the dashboard
2- the valve one the bulk head
3- the route your cable takes
Be warned though even working correctly they can be stiff.
1- Undo the heater knobs with an Allen key and pull them off (don't drop the grub screws, you will never find them). Unscrew the four screws
holding the centre console in. It will then fall forward enough to allow you access to the control mechanism. A quick spray with WD40 and some
wiggling should free it up. I always grease mine as well.
2- The valve on the bulk head is easy to check...it has water pipes going in and out of it, undo the cable and see if it moves, they are stiff
but can be a little delicate so don't force it too hard.
The heat coming in to the car may not be from your heater, check it by closing the valve on the bulkhead locally (undo the cable). My cockpit
gets very warm especially by my feet. It's because I have a four branch stainless exhaust, which increases the heat under the bonnet and runs
along the side of the driver. see this thread as well
http://www.tssc.org.uk/messageboard.asp?sf=110&boa...
Check that your seal on the gearbox tunnel is alright then make sure you have enough screws holding it in. You can put extra shielding in there
as well.
Hope that helps Jon