WRabbit writes

Best bet might be some bush roses. Ideally, you would prune them down
to about a foot each year (thus temporarily losing your protection),
but you could experiment with pruning half in alternate years.

Considered this but there's a couple of issues. As well as discouraging
them from sitting on the wall they've got a habit of clambering over it to
retrieve their ball (several times a night). The coping stones are getting
knocked loose. Don't know that roses would deter them enough.

Depends on the rose. HTs seem to have long smooth stems with the
occasional stout thorn - not much of a deterrent. I have a bush rose,
possibly floribunda type, with lots of deep red roses and - the
important bit - a growth habit which is a tangle of twisted branches
covered in small spines. It certainly keeps me away from that bit of the
garden, and I've never had anyone barge through it

In addition
the rose I planted at the back suffers quite badly from blackspot.

Some are more susceptible than others.

We'd be planting on the north side of the wall, and the area does
get quite damp, so we'd need to improve the drainage there (digging
some sand through?)

If it's damp and clay, add humus (garden compost for example). What do
you mean by 'quite damp'? - in my garden 'quite damp' means 'standing
water after heavy rain' ;-)

Grass slopes slightly done to the wall, not quite swamp like but certainly
muddy after heavy rain.

So where does the water drain through after that? Improving drainage
only helps if there's somewhere to drain to.

Raising the level of the ground near the wall by a few inches might be a
possibility - improve the damp problem and give the hedge plants a few
inches less to grow ;-)

Since you've said you like holly, I'd be looking at holly in the long
term, with a short term solution while the holly were making their
growth - quite what the short term solution would be, I don't know. Wire
netting along the top of the wall? Nurse crop of stinging nettles? ;-)

--
Kay