I have a strong feeling that you may have what is termed Gyrodractylus or skin flukes or Dactylogyrus gill flukes .
When a fish is infected with skin flukes it will start to produce large amopunts of mucus as you see with your fish in the photo's if the infection becomes severe then secondary infections may follow such as bactrial problems or fungus .
With gill flukes if they are a problem then a discolouration of the gills can be seen and the presence of large amounts of causing the gill fillaments to stick together ,
If this mucus builds up then the gill cover [operculumn] then it gives it the appearance of being lifted and the effected fish hang or parks near to heavily oxygenated water i;e waterfalls or returns to the pond and become emaciated followed closely by secondary infection of which Malachite and propolis should be used to treat these secondary wounds with .
Seeing as your from the UK go ask your local dealership for a packet of Kusuri Fluke M and pay attention to the instructions on the packet
However switch off any protien skimmer prior to use
Directions shake contents of sachet well messure out 11.8g per 1,000 gallons imperial disolve into bioling water allow the hot mixture to cool before adding slowly to the ponds return over a 0ne hour period ,
This treatment is designed to be a one off but it may well require a second dose after 7 days keep a close observation of your infected fish throughout the treatment .
Dave