On the 147 there's a weird, contorted bracket for various pipes and wires that bolts to the thermostat body, using a single short bolt at an odd, 45deg upwards angle - it confused the hell out of me when trying to figure out how to dismount the stat. Maybe your 159 is similar - that looks like the mounting hole on the bracket itself in the middle of the pic, so not yet mounted to the thermostat. From memory I think the bolt is M8 x ~15mm.
Previously I said I didn't think there was a bleed screw for the cooling system. That's wrong. On the 147 I found there is a coolant bleed plug, central top surface of the rigid metal water pipe that runs across the front of the engine. That should be opened to allow trapped air to escape once the system is filled with coolant. Whether the 159 has similar I don't know, that pipe is a different design, from photos. If you do have a plug, open it with the engine cold, until coolant begins to emerge, then tighten.
Sounds like Pud had a spontaneous bonnet opening incident whilst driving
That's a known fault on some Alfas, where the catch fails to lock properly. I know it's a hazard on the 147, so probably 156 also, maybe others too, with the recommendation that the catch is kept clean and well lubricated, and any time the bonnet is closed, special attention be paid to ensuring the catch is properly engaged. IE try lifting the bonnet to ensure it's latched properly.
I had the same thing happen at 70mph on the A2 c.1972, in my first car, a Hillman Superminx convertible. Couldn't see a damn thing, but aimed for a layby, went onto grass and was very lucky that I got it stopped about a foot short of a giant metal roadsign I couldn't even see. Most unpleasant. New bonnet, hinges and pants required.