I spent ages researching the symptoms and with the 'lump' not being cottonwool-like, but more like a cyst, Nodular disease was the only one that kept fitting the bill.
Below is the info I found....

Nodular disease
Signs Of Infection
White lumps or boils appear anywhere on the body.
Details Of Infection
Caused by Sporozoans, single celled spores that can multiply and spread rapidly. The spore's life cycle usually begins by ingestion to the intestinal tract by the fish. Here the spores infest the bloodstream penetrating internal organs before forming visible cysts and boils on the fish's body. When these boils rupture, millions of spores are released and so the cycle begins.
Treatment
Unfortunately chemical remedies are frequently ineffective and the only recourse is removal and humane disposal of the infected fish.

Nodular disease
Sporozoan infections cause cysts (containing the parasite) varying in size from pin-head to pea-size spots. They affect a wide variety of fish. Some affect the balance of fish they infect.

....Apparently the spores are found in tubifex worms...which i used as a feed