1995 Vauxhall Cavalier cavalier quickly overheating in slow traffic

M reg (1995) Vauxhall Cavalier, 2.0 SRi 16 valve, Single overhead cam Ecotec engine. Approx 137,000 miles

[b]Symptoms:[/b]
* Car temperature is OK on dual carriageways and motorways
* Car quickly (1-2 minutes) overheats when stationary in traffic - I can see the needle moving!
* Seems to be OK if I'm moving in 3rd, 4th, 5th gear and revs are above about 1800 revs
* Putting the blower on full fan and full heat helps to keep the temperature down when in 3/4/5th gear
* Interestingly, the air coming into the car from the blower can occasionally go cold and then the car starts to overheat. I presume there is a heat exchanger for the internal temperature control. Does this heat exchanger have it's own thermostat control?

[b]My feeling is that the overheating problem is being caused by a sticking / stiff main thermostat since:[/b]
1/ The radiator fan doesn't come on. I'm assuming that if the thermostat is shut, the water in the radiator doesn't heat up and therefore the radiator fan temperature switch doesn't register the engine overheating. I haven't been able to test the radiator fan though (see question below)

2/ The engine temperature sender unit is actually part of the engine and so (quite rightly) measures the temperature of the water in the engine

3/ I think that once the revs get about 1800 revs there is enough pressure in the system from the water pump to force the thermostat open so that water can flow to the radiator for normal cooling eg on Motroways etc

Any comments on my thoughts above?

[b]I have two other specific questions:[/b]
1/ I can't test the radiator fan temperature switch since I can't get the connecting plug appart - any ideas?

2/ I think I've located the thermostat on the front left of the engine near the timing belt. Looks simple enough to remove. Main hose from radiator and then two bolts. However, on closer inspection of the hexagonal bolt heads, the straight edges have dimples / cut-outs in them. Does anyone know if it's possible to use a standard socket set on these or do I need a special tool?

Thanks in advance for any input.