You know it won't travel don't you!
You really need the Keo bitters and the Lanitas lemon syrup rather than squash. I've managed a passable imitation using the french lemon syrup - the stuff that comes in tin containers. Otherwise try lemon squash and lemonade instead of soda water.
All you need then is 25 degrees in the shade and a party that starts at lunchtime. Oh - and a 7 course meat feast.
I found some references on google but most have advertising connections:
3 to 4 splashes angostura bitters swirled with ice then poured out
2 measures of brandy
1/2 measure lemon juice
topped up to taste lemonade