Adjusting the position of tempo indications

[David Sumbler]

The default position of tempo indications is, to my eye, rather too
close to whatever is beneath them, be it a stave, a note or a slur.

I have tried experimenting with

\override TextScript #'padding = #4

and

\override TextScript.padding = #4

(I was unsure of the syntax) just to see if I can get my tempo markings
to move, but neither of these works.

How can I get tempo markings to be placed higher?

Also, occasionally I have two such markings quite close together
horizontally.  How can I get them both to be placed at the same level,
even if this means that one of them has more than my default level of
space beneath it?

David