We was only talking about chain failure yesterday, mentioning how they can pop off the cogs due to burnt, or clotted oil clogging the guides or tensioners but the chain does not actually snap. I've always been dubious with the oil specified too. Alfa say use 5w/40 oil, Vauxhaull use 5w/30, Saab GM use Mobil 1 0w/40 in the USA. The main reason for chain failure due to oil contamination is not changing the oil & filter regular enough, once per year may not be frequently enough. It comes down to how you use the car, example by doing sort trips, popping to the shops, stuck in traffic on the commute to work, hard driving, contaminates the oil far quicker than you may think. Cars that do lots of miles in one trip can leave the oil change for much longer, so anywhere between 3-10k oil & filter changes maybe required.
I'm not trying to patronize anyone, but wasn't the oil gague showing signs of overheating due to the oil struggling through the oil pump, or wasn't it making any noise prior to the fault codes & chain actually popping off?
You can see the oil is burnt on the engine & will need a scrape to get the treacle stuff off so the new chain can run free once a new ones fitted. To me i'd say the oil wasn't change enough or checked for viscosity when pulling the dip stick. You'd probably best off getting a Vauxhall Oil pump than wait for Alfa to sourse one, it's not a Fiat/Alfa made part hence the long wait, you'd get a GM Vauxhall one from probably europarts or ebay right away, no bother