I understand the heater control knobs & head light switch knob should pull off to enable bulb replacement behind these?
I've pulled & pulled & can't remove them. Any tips please?
Should I gently heat them with a hot air gun? Should I squirt WD40 penetrating oil behind them? I don't want to damage them.
Also, am I right in saying they should be 0.4volt ?

It would have been nice to know what car/model your are talking about.
If it is a Yugo, I really can't help you If it is a W124 Mercedes, my answer would be different.
JackD

Sorry. Forgot to mention model. It's a 1994 320 TE (W124).

Headlight swith: The headlight switch has a bulb behind the panel. To access the bulb you need to remove the knob. Simply pull on the knob, it will come out. If not, pull harder.
Once the knob is removed, there is a nut behind the switch. remove and slide the dash panel to the side. This will uncover the bulb.
Heater control: You first need to remove the wooden section around the control unit. 2 screws on each side (looking up) of the radio. There are 2 more screws under the wooden section, on each side of the heater control unit...remove.
You will need a pair of baby fingers to get the the light bulb.. A pointed pair of pliers could be usefull (do not twist, simply pull out)
To re-insert, use a straw or a piece of vacuum tubing (1/4in).
Good luck
JackD