As I start with all threads, we like to start with water quality numbers for ammonia, nitrite, pH morning and evening, and KH as the fishes environment is primarily responsible for the fishes health. Spot on doesn't really tell us much.
With your temperature, approximately 52F, you are borderline on being able to heal the fish in the pond. I would recommend a QT tank of appropriate size for wintering over, with sufficient filtration to keep pristine water. The QT tank needs to be maintained at a minimum of 20C with 25C being better.
The fish needs antibiotics, and I don't know the availability of injectible antibiotics like Batril or Amikacin, and our favorite in the US isa powder Tricide Neo, which is also marketed as Neocide 3 but being a US product, not sure of availability in Scotland. Barring any of those being available, the use of a Triple Antibiotic Ointment would be of some help.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard