There are two songs with that title that I know of.

One is the Stanley Holloway version - written by Sidney Carter.

DOWN BELOW^^
(Sydney Carter)

When you're working in the dark... Down below,
Underneath St James's park ... Down below,
When you're working in the dark, oh it isn't half a lark,
When you're working in the dark... Down below,

It isn't hard to tell... Down Below,
If it's Bow or Clerkenwell... Down below
For Bow and Clerkenwell have a different sort of smell,
And we know them very well... Down below,

Over Covent garden way,... Down Below,
In the merry month of May,... Down below,
The fragrance of the flowers gives us many happy hours,
And we sing a roundelay,... Down below,

The objects that we find... Down below,
Help to entertain the mind,... Down Below
There are watches that won't wind, wrapped up in bacon rind,
And that isn't all you find,... Down below,

When you're under floral street,... Down Below,
With the water round your feet,... Down below
Mid the cabbages and beet you may find a marguerite,
And the thought is very sweet, ... Down Below

Hatton Garden is a spot,...Down Below,
That we like to go a lot,... Down below,
Since a bloke in Leather lane, dropped a diamond down a drain,
We've been waiting... But in vain,... Down below,

When to billingsgate we come,... Down below,
When to billingsgate we come,... Down below,
When to billingsgate we come, You can tell it by the hum,
And we wish we hadn't come,... Down below,

There is something in a sewer,... Down below
That has a strange allure,...Down below,
The magic of the drain is a thing you can't explain,
But it's calling us again,... Down below.

The other is one that Bat Goddess sings - part of the lyric is

It's the only place I find I can get some peace of mind, I'm glad I'm working down below.