Reading your symptomsI would be checking the level of the coolant in theheader tank. These tanks are famous for leaking in the earlier cars and if the level drops can cause your problem.

 

The other area is the coolant control valve although that by default fails open (hot), it can stick closed causing no heat on one or both sides.
One other cause can be the external temp sensor which is another standard fault, it normally gives a high external temp reading on failing which confuses the system into thinking no heat is required, if you do not have the external temp selected on the display you could miss that.
Some owners have reported wiring faults to the heater control valve.

There is ascrew in the pipe at the front of the engine but anyone thatI know that has gone near it has eneded up breaking it. I just bled mine after changing the tank by running with the cap off and the heater selected to max.

 

Probably your cabin heater controlvalve is choked with gunge, well documented s type problem. I bought one off e bay and had mine done locally but I would have the coolant tank checked for cracks as these are a well known fault too. Mine was cracked on two sides when it was checked. Also got replacement off e bay. Not big jobs but worth doing both at the same time as coolant has to be dumped.

 

There is a hose attached to the side of the expansion tank that has the bleeder screw. It's usually tucked away under the edge of the wiper cowl down by the tank. You can also remove one of the throttle body coolant hoses for a more complete bleed.