This is quite a common one, likely causes are:
Split in inlet trunking; that long black pipe in your picture. Take it off and check underneath as that is the mostly likely place for a split. Can be bodged with some extrior grade silicone but really needs replacing.
Idle speed actuator; slightly visible behind the throttle body. Easy to remove but don't loose the O ring. £80-£90 for the part from memory. ECU will probably require reseting of the target idle after replacement.
RPM sensor: round the back of the engine but can just about be done from the top. £10 part from memory but again will probably require ECU reset.
However, comedy tickover can be a bit of a one to diagnose, as just about anything that upsets a sensor may cause it, by giving the ECU false information. If you search the forum you'll see lots of posts where people have spent considerably more time chasing the problem than actually solving it, which is sometimes really easy when you get to it. Mine was the £10 RPM sensor but.......
Consider buying diagnostics. They tend to pay for themselves very quickly, compared to a dealer.
Most common is AlfaDiag which runs on a PC normally via a USB port. You can buy the cable to convert the EODB socket in the cockpit to USB.
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