The poem is Sympathy in White Major

Here’s the first verse.

When I drop four cubes of ice

Chimingly in a glass, and add

Three goes of gin, a lemon slice,

And let a ten-ounce tonic void

In foaming gulps until it smothers

Everything else up to the edge,

I lift the lot in private pledge:

He devoted his life to others.

Larkin would’ve probably used Schweppes tonic. In 1967 a “go” (spirit measure) was usually 25ml, 35 ml in some establishments. 10 fluid oz is 284 ml. So Larkin's recipe is probably 75ml gin plus 300ml tonic. I used 50 ml gin and a whole can of tonic (150ml) for each one.