Thanks again for your help Pud.
So today I got a little more depressed with the Alfa...
Fitted a new Dayco tensioner (eurocarparts - £100), noise improvement but still present.
Decided my eBay 6PK1105 wasn't upto the task as it was flapping about a lot on the power steering to crank run.
Rang up my local motor components, they had the 6PK1103 belt, which was apparently the correct one for my car (looked in the book myself) so they relieved me of £14 and away I went.
The belt does feel a lot more solid, but ultimately I fitted it and now the rattle is even worse.
Power steering + alternator pulleys are good, no play or drag.
Crankshaft pulley is new + looks straight and true.
New dayco tensioner fitted
New Gates 6PK1103 drive belt fitted.
Still same noise that makes my car sound like a shed.
Any advice from here? I don't know what else to do....