Sausage discolouration - is it safe to eat?
My question is: what causes this discolouration 


Many meats are dyed to make them look fresher. They add red, cause meat is red right? Well no. Most meat will "brown" or "gray" as it is exposed to the air and the blood dries up (or drains out). This is not, in any way, a sign of bad meat. In fact it "may" be a sign of good, natural meat. 

A YouTube Video that explains it 


  and, have I needlessly thrown three very tasty looking sausages in the bin?


Yes, probably. 

Important note

When in doubt throw it out. A week in the fridge (not freezer) is border line for me. I would have probably cooked it, but I wouldn't fault someone else for not cooking it.  

You can usually look for smell or slime to tell if meat is bad.