Ended up mullering the plug by using a slightly sloppy allen key; so I was looking for a replacement and the factory one is quite expensive £10.74 + del
I found a Stainless Steel version for a good price on eBay.
STAINLESS STEEL CARC REAR DRIVE FILLER PLUG FITS MOTO GUZZI GRISO 1100 / 1200: Search Result | eBay