Dropsy is not actually a disease; it's a symptom of a disease, most often bacterial. It would help if you could fill this out;
Tank size:
pH:
ammonia:
nitrite:
nitrate:
kH:
gH:
tank temp:
Fish Symptoms (include full description including lesion, color, location, fish behavior):
Volume and Frequency of water changes:
Chemical Additives or Media in your tank:
Tank inhabitants:
Recent additions to your tank (living or decoration):
Exposure to chemicals:
Digital photo (include if possible):
I could recommend meds, though the meds I usually use aren't available in the UK. If it's bacterial it certainly is contagious, and a separate tank would be best. It doesn't have to be a tank, many people use large Rubbermaid containers or a bucket for a hospital tank.
Dropsy is a tough one to treat; I've used antibiotics & epsom salt to get a 50% success rate.