Andy wrote:

The lily came with a few leaves on stems about 3 inches long so I'm
guessing it's been grown in shallow water.

It doesn't mean that. Lilies will usually have one or more "indicator"
leaves, which don't grow much but trigger the full growth when light levels
get strong enough.

I want it to sit at around 20 inches in the pond and it's been there for
a couple of weeks with no sign of growth.

You'll get the most rapid development if you can set it so that the leaves
are just a couple of inches below the surface, then lower it a few more
inches every time leaves reach the surface.

I read on one website that the best method to encourage growth was to
trim the mature leaves off as the new shoots would grow to the surface.

You don't have any of those.

If so, where so I make the cut? Just before the old leaf or right back
at the base of the stem?

I always remove mine as close to the base as is easily possible - the whole
stalk will rot anyway.

Oh, one other thing - how fast should I expect the lily to be growing?
I'm in the UK with a small pond around 7x5x2ft.

It's way too early, as far north as you are, to expect much growth yet. In
July and August, if you have lots of sun, you can see 6" of growth a day
(actually, a leaf or flower bud 6" below water one day will surface the
next, but that may be at least partly because the stalk straightens out).
--
derek