Astra radiator fan question

1996 Astra 1.7TDS 5 door hatch. Is the switch for the cooling fan on the rad or the head (or elsewhere) ? The fan didnt seem to be working during the recent hot spell and temp gauge went v high but no boiling. There is a large(ish) switch on the rh side of the rad with 3 wires which I presume is the temp gauge sender. Also a small switch with single wire on the lh end of the head which I presume is the fan switch. Shorted this wire to earth but fan didnt start. Checked fuses, all ok. Is there a relay for the rad fan, if so any idea where it is located? (LH & RH are as viewed from the drivers seat) Didn't get more time to look into it, sons car and he was back off to Bristol after a flying visit. Any help or advice would be much appreciated. I could be overlooking something very basic. Many thanks Nic

The fan switch is the three-wired big one on the radiator, the temp sender is the little single-wire one on the head.

The fan wouldn't normally cut in while driving along as airflow through the radiator is usually enough to cool the engine. Also, some fans on Vauxhalls don't cut in until the temp gets real high, near to the gauge's red area. I only hear my Daewoo (yeah OK, I know!) fan in heavy traffic or stationary idling and the same applies to the Corsa, Astra and P205 we've had.

Follow Mr Cheerful's advice to check out the fan and switch if it isn't cutting in before the temp needle gets to the red area when standing still but suspect blocked rad or slow/sticking thermostat if it's getting hot when driving along.