In sun or shade? How high is the wall?

Hi, it depends on the height of the wall, Hypericum hidcote grows to about 4ft high, while lavender, depending on which species you get, can be anything from a few inches, up to 4ft, also with most lavenders, once you cut the flowered stems off, the height is reduced by about a half - 2/3s, and looks a bit uninteresting in winter, whilst the Hypericum would be evergreen, or near evergreen, Derek.

I think the hypericum might lose most of its leaves in a very exposed position? Those it keeps aren't pretty in the winter anyway, if that matters.
Would the wall shelter the hedge?

In my experience, Derek, Hypericum Hidcote gets at least 6 feet by 6 feet even with regular pruning.

Hi Bamboo, that must be the difference between north and south, I've never seen it any more than about 4ft, but it still needs regular pruning, lol, Derek.

We had some where I used to work and they were cut down to about two feet every winter. They lost all their leaves (not surprising but they still flowered well. One down our road had reached 6 feet. Mine's reached almost five feet in five years.
Pity Mrs Whiteford hasn't come back with info on position and also where she lives. Difficult to help without it.

Well that's interesting Derek, maybe you're right about the north south thing...