Thanks for your reply McKoi

$$ How large is this pond? Is it filtered? Is there a lot of mulm on the
bottom?

It's about 2,000 gallons, 3ft deep. Filtered with UV, fairly clear
water. Test results are all coming up good, PH, nitrites, etc. Well
stocked with oxygenating and cover plants. Stocked with 16 fish of
various types (Orfe, Red Goldfish, Shubunkins, Ghost Koi, Mirror Carp
and a couple of small Tench).

$$ Could it be the dreaded ulcer disease? Koi-Zyme and some antibiotic food
should help - and very clean water.

It doesn't look like any pictures of ulcers I've seen anywhere, which
is why I wondered if it was new skin/scale growth or some form of
necrosis of the damaged tissue.

$$ Not if the pond was the problem in the first place.

All of the other fish in the pond are perfectly healthy, no signs of
disease, lively and feeding well.

We did have a problem a few weeks ago whereby, whilst we were out for
the day, the charming children at a birthday party next door decided it
would be fun to throw the remains of half eaten chicken skewers and a
few of their toys over the fence straight into our pond. The skewers
were found near the waterfall, where the fish like to play in the
running water. Needless to say, the fish were a bit timid for a while
after that. I'm pretty convinced that this was part of the problem,
leading to stress, possible injury and thereby the dreaded fungus.

I've subsequently treated with salts and am going to add some stress
coat today.

Any further suggestions will be appreciated.