I noticed this huge slug in my garden that had a large hole in it's side so i decided to look it up
Could this be the Red slug (arion rufus) or the Black slug (arion ater)
The colour of the black slug is generally black, but the colour can vary. The general trend is for a darker pigmentation the further north the species is found. The classification of brown coloured and rust coloured "black slugs" is some what disputed. The brown variation is sometimes considered to be a seperate species (arion rufus) the red slug, Young specimen's of black slug do have a brown colour which is later lost if and when the slug changes colour to the mature state.
The hole in the side is called a pneumostome a breathing hole on the right side of it's mantle.