Decided to tackle the rear discs at last. The old ones were not too bad, but I had already resolved to do the fronts so I thought I'd do the rears while I was at it. Sooooo.. 1) Loosen the wheel nuts and jack her up. Put her on axle-stands and remove the wheel.
Push out the pad retaining pins using a suitable punch (or I use a three-inch masonry nail). The rattle spring will drop out. Work the pads loose by firmly pushing the away from the disc to push back the piston, and then wiggling them free.
Undo the two 17mm bolts on the back of the calliper. Be carefull here not to let the calliper drop off (some hope) when you remove the bolts. There is no flexible brake line here so try not to over-stress the hard-lines.
Manoeuvre the calliper free of the disc and support it securely so that it won't fall and damage the brake line. Here I have it cable-tied to an axle stand set at the same height.
Then undo the two retaining screws that you can see in the top photo and, in the words of an Alfa manual, "separate the disc from the hub". As I've stated previously, this phrase usually means that a lump-hammer and a certain brutality will be required in short order. Interestingly in this case one side simply slid apart; the other, unfortunately, was a right bugger to get off! Oh well, I got there in the end.
Anyway, after that it is just a matter of degreasing the shiny new disc with petrol or whatever and then mounting it to 'ooh's and 'ahhh's from the assembled throngs.
So refit the callipers and use new brake pads. If you use the old brake pads you'll just wear the new disc to the same wear pattern as the old one; it's just not worth it!