I have a Volvo V40 with the 1.8i engine. This engine is identical with the Mitsubishi 1.8 GDI engine.
You can clean the throttle house yourself by following the steps below.
The throttle house is attached with four bolts, two cooling lines and two electrical connectors.
On the Volvo (also on Mitsubishi?) the throttle house is mounted behind a plate, which is attached with three bolts.

This is how you remove the throttle house:

Remove the three bolts to loosen the plate in front of the throttle house(if needed).
Remove the uppermost coolant line
Remove the uppermost electrical connector
Remove the four bolts holding the throttle house
Loosen the throttle house and remove the second electrical connector and coolant line.

Now the throttle house is detached from the engine and can be cleaned.

I used a product called 'Forte Air Intake Cleaner' to do this job as it was recommended by a Mitsubishi representative.
I am today convinced any 'thinner like solvent' will do the job just as well.
Remove ALL the soot in throttle house, since remaining soot might act as nucleation point to new soot formations, and you dont want to do this cleaning more than necessary.

When you're done, mount the throttle house on the engine again.

When you start the car, you probably find that the idle is far higher then normal. This is because the idle adjustment is no longer correct after the cleaning.

This is how you adjust the idle in a 1.8i or GDI engine:

Disconnect the electrical connectors from the car battery for a few minutes (this resets the onboard computer)
Connect the electrical connectors to the car battery again.
Start the engine and make sure that all electrical appliances in the car are off.
The engine will now adjust the idle rpm automatically.
Leave the engine to run for 10 minutes
Stop the engine and start it again, but this time with the airco swithed on.
Leave the engine to run for 10 minutes
Stop the engine and, voila, the idle rpm is adjusted!

The complete job took me 3 hours the first time i tried it.
The next time (in three years time according to the garage) I reckon it will be done in about one and a half.

Good luck!