Update:
I called the dealers yesterday, they only sell the new metal version, but confirmed it was a different thread to the original so a new rail would need to be fitted. This would be £130(something)+VAT for the part.
Disappointing that ALFA have obviously recognised a problem and changed the part for a more suitable one, but also changed the thread. Effectively, there is no replacement for a shoddy part and seems like a way for them to make a bit of cash :tut: on top of that, there's a metal one in my 156 2.4 JTD!
I ordered the plug (from the link i posted earlier) which was £2 + a 20p washer, fitted perfectly and doesn't seem to be any leakage, so hopefully job done for £2.20! Quickest and cheapest fix i've ever done i think. The remaining plastic that was in there from the original plug was easy to get out, just wiggled a flathead screwdriver in there and unscrewed it.
Again a MASSIVE thanks to everyone who has helped, especially on a Saturday night. My breakdown hadn't transferred over and so I was pretty much stuck. :thumbs: