Cooling Problem?

[Paddy O]

I noticed that after a run the radiator fan no longer kicks in for that few seconds when I turn off the engine, instead I can hear gurgling & hissing from the expansion tank - & now steam
The temperature gauge is always in the middle when warmed up and she drives as well as ever at 156k - I have never been able to hear the cooling fan so I don't know if it kicks in at all.... it ain't blown up so I guess it does.
So today I took her for a run, gauge shows normal, lifted the bonnet - & the cooling fan didn't come on at all.

I suspected the coolant temperature sensor (9186486) but it shares with the temperature gauge (?) I think, which works.
Disconnect the sensor wires and the gauge stops working but the fan turns (still nice & quiet I must say), until the engine is re-started.
I'm a competent enough mechanic but this has made my minisule brain hurt.
Is is a bad relay do you reckon?
And you you guessed it, does anyone know the part number if it is.
Help please folks.

[Ron@iPd]

If you turn the A/C on does the fan come on? If so, the relay and motor are good and the temp sender is most likely bad. If you turn the A/C on and the fan does not turn on the relay or the fan motor are bad. Just a thought.

[Paddy O]

No A/C mate but thanks.