Some fish are more prone to certain parasites than others.
Camallanous worms are usually seen in livebearers but can affect other fish.
Get you some info.
The description you gave does sound like camallanous worms.
Do you have any livebearers in the tank.
I left a link to the sera med seen as you are in the uk.
You have to buy it online.
Camallanus
Symptoms:
When the fish is stationary, deep red worms can be seen protruding from the anal pore. They are only visible when the fish is still because the worms retreat into the intestine at the fish’s slightest movement. Other signs may include an inflamed and enlarged anus. In severe infestations, the fish may become emaciated and spinal curvature may also occur.
Cause:
The parasitic worms Camallanus Cotti and Camallanus lacustris. These small, livebearing parasitic worms attach to the intestinal walls and rectum with pinchers. The worm’s grip is so tight that any attempts to remove it forcibly will rip away tissue from the intestinal wall. The worm’s pinching causes ischemia (reduced blood flow) to that part of the intestine. In time, the tissue to that part of the intestine will die, at which time the worm will migrate to another part of the intestine. This causes perforations throughout the intestine, which allows other pathogens to gain entry. If the fish is not treated, either the parasite or bacterial will kill the fish. Camallanus infestations occur most often in livebearing fish such as Guppies and Mollies, though infection is possible in all fish. Camallanus infestations are contagious. All fish, including those not yet showing visible symptoms, as well as the aquarium, should be treated.
Treatment: United States
Treat with Internal Parasite Guard, Pipzine, Disco-Worm, Trichlorfon or Fluke-Tabs as well as a medicated food to prevent a bacterial infection.
Sera Nematol (UK)
Sera Nematol is a very effective remedy against nematodes such as Camallanus, roundworms (Capillaria) and discus pinworms (oxyurids) for fresh and salt water fish. A sensational R & D success: sera med Professional Nematol is the first ornamental fish treatment against Nematodes that may be sold freely. It is based on a patented Emamectin/solvent complex, highly effective and easy to dose. sera med Professional Nematol is particularly effective against Camallanus and round worms of the genus Capillaria, which mainly occur in angelfish and discus. It can also be optimally used against pinworms (Oxyurida), dreaded by discus keepers. At the same time it combats the small copepods that Camallanus uses as intermediate hosts. Result: With sera Nematol a well tolerated and highly effective ornamental fish treatment that can be sold freely is now available for the first time! It is a liquid and therefore can be dosed easily and precisely.
Dosage for sera Nematol:
Add 1 ml sea Nematol per each 40 litres of aquarium water. Distribute the treatment on the watrer surface from the dosage cup while stirring it into the water with a spoon or staff. Aerate the aquarium well during the treatment, switch off UV-lamps and do not filter through active carbon. A water change of at least 80% must be carried out after two days (or during the treatment if the water gets cloudy). You can combat larvae that have hatched from the eggs in the meantime with a second application of sera Nematol three weeks later.
From £12.99
/www.inkmkr.com/Fish/FlubendazoleTreatment.pdf) I have just emailed the guy to see if he can supply me with the treatment. These worms sound like a NIGHTMARE. I'm not happy. If these things end up infecting ALL my fish.. then I'm giving up 
