Where is the return spring situated on my 2004 Holden Cruze auto

I presume you refer to a 2011 Chevy Cruze?
If not this fix applies to most automobiles.

The release lever is located just inside the door low under the wheel and gauges near the kick plate. Reach down under the dash and pull up in the lever to release the hood.

One the hood is released, pull up on it And push the catch until the hood raises freely and then lock it open with the hood prop located inside the motor compartment.

https://www.youtube.com/watch?v=uU9DJOeVcLQ

it has done 48,000 klms per year which indicates that it is not a suburban car ( averages 20,000 klm /year)
That indicates that it is a commercial / travelling representative car or government car
you would be well advised to have it check by an auto club inspector (RACQ, NRMA , RACV or the state where you are) and get a comprehensive report
google -- reports on holden cruze cars -- and read forums from users and owners

Spanner sign means major service .

you don't, hand brake is not adjustable. If hand brake does not hold something else is at fault. Possible streched cable however it would not be unusual for the handbrake to be high but it should hold vehicle on a hill.

replace the cable first and then check for a missing return spring

the cost will be a replacement radiator and an engine coolant flush out both which can be done at an accredited radiator shop
if the oil in the auto transmission is pink ( should be cherry red or clear tan depending of oil used) then there will be a transmission fluid flush as well required
both operations will be considerably less that another car
the problem is that the oil cooler in the radiator tank has split allowing transmission oil under pressure to contaminate the coolant
with any sort of luck the split will seal off when the engine is stopped thereby stopping coolant from getting back into the transmission

Fluid that is calld for by maker. U need a pressure chec done on trans..if ok then theres mech issues..may need seals rebuilt or new torque converter

We don't have this make here, please give me engine type and if any US care is like this, I know that Holden is really just GM