Gin

Where I worked, two places had very similar recipes close to this one. But the joy is make it...change it to how you want it next time. Dont like the basil? Throw it. Many places dont put it in...but I like it.

Dont do the whole leave it aside for a few days: just make small cubes of the ccumber and add it to the boston (or mixing glass). As for celery bitters, you could vary and use a couple of large mint leaves and a drop of ordinary bitters if you have some to stir in with the mix to give it the herby bitter edge.
Simple syrup is basically as much sugar in water as you can get. Play with the amount in the drink as you make/taste it to suit what you have made.

Recipe: English Garden Cocktail, inspired by Gordon Ramsay Steak
By: Web Gangsta

Summary: This is our version of the English Garden martini cocktail that is served at Gordon Ramsay Steak at the Paris Las Vegas resort

INGREDIENTS
Hendricks Gin
Celery Bitters
1 cucumber
simple syrup
Fresh lemon juice
Fresh lemon zest
Fresh basil leaf
Fresh ice
INSTRUCTIONS
Peel cucumber and cube into 1/2-inch sized chunks.
Fill a sealable glass bottle or other container with the cucumber chunks. Pour gin into container to cover cucumber, then seal and place aside for a few days
When ready to serve, chill cocktail shaker with ice
Add three shots of cucumber-infused gin to shaker
Add 1/2-shot of simple syrup to shaker
Add one shot of fresh lemon juice to shaker
Add six dashes of celery bitters to shaker
Stir lightly to combine ingredients (do not shake).
Strain into martini glass that contains a basil leaf and one or two of the infused cucumber cubes. Garnish with lemon zest
Sip and enjoy!

A more home friendly version and closer to what I remember as the original before it got tarted up. Add all ingredients to a glass filled with ice, stir untill it is diluted enough to your taste and strain in to a cocktail glass. You may want to add some sugar syrup (even honey may go with the lemon and apple..youll have to try it) to take the bitter lemon and elderflower edge off.
I personally would try adding a basil leaf floating on it for garnish (as fragrance is half the taste) and take it off if it does not go. One cucumber slice for garnish.

All I say is experiment, cocktails are like art/cooking...its not a strict set of rules.

2 shots Hendrick's Gin
2 ½ shots pressed apple juice
1 shot elderflower cordial or St-Germain Elderflower Liqeur
½ shot freshly squeezed lime juice
Cucumber slices, for garnishing