In article , StanDan
writes

The wall is damaged but too expensive to repair due to it holding up a
higher level of garden.

Good heavens, if it's only three feet high, surely you can repair it
yourself . . . or at least have a good go at it!

I want to cover the wall up

You mean the damaged one?
Surely if it is holding up the upper terrace, it will be pointless
trying to cover it up as the upper terrace will eventually fall out
through it?

and have dug out 8" deep gulley 1' wide along
the base of the wall

So, have you gone below the wall foundations?

with the intention to make it into a bed. I intend to
put a layer a lawn sand in the bottom

Why on earth lawn sand?

then a layer of compost then topsoil.
The rest is THICK clay.

I then want to plant some EASY CARE plantlife that will provide some colour
to the garden and that will cover the wall quickly!

I think you are going about it the wrong way.
You want stuff growing *down* it.

If I were in your shoes, I would have a bash at repairing the wall
myself. Then every so often I would build a rock plant in that would
grow downwards. I would also plant lots of aubretia, alyssum, phlox etc
and possibly some ivies at the top of the wall because it is far easier
to introduce a good amount of compost/sharpsand/soil at the top of the
wall than the bottom of the wall and it will drain far better.
--
Jane Ransom in Lancaster.
I won't respond to private emails that are on topic for urg
but if you need to email me for any other reason, put ransoms
at jandg dot demon dot co dot uk where you see