To follow this cocktail's original proportions use 60ml (2oz) dry gin and omit the Old Tom, but you'll be missing out. Either way, despite the rich liqueur...
I made this with St. George Valley gin (didn't have any Old Tom) and Cinzano. I felt like I couldn't quite get the taste right; I tinkered with amounts of lemon, Grand Marnier, and finally added a bit more gin and that seemed to do the trick. Next time I'll lean a little heavier on the gin in the beginning.