Hi Michelleuk
When I first saw your post I wasn't sure, but now I think it's a pretty sure thing that your cory has a bacterial infection. This often happens when a fish's immune system has been weakened to the point that bacteria that's not normally harmful can hurt it.
Some of the stress factors that can cause this are infrequent or inadequate water changes, low oxygen content in the water, temperature fluctuations (especially fast warm ups) aggressive tankmates, high nitrites or nitrates, uneaten food in the substrate, overcrowding or inappropriate water temperature. The first thing to do is correct the situation before other fish become infected too. A big water change and bottom vacuuming is the place to start.
If you can quarantine him for treatment, so much the better. Since you're in the UK, I would suggest using either Waterlife's Myxazin or Interpet's Anti-Finrot & Fungus medicine to treat him.
During treatment it's good to do regular water changes and replace the medicine as necessary. This will help resolve the problem too. If you can keep the temperature at no higher than 25 degrees C. it will help too. If you have less than good water movement at the surface, an airstone will help by adding more oxygen to the water. This will make the environment less hospitable to the harmful bacteria and help your cory to heal.
Good luck. I hope he gets well soon. Please let us know how he is coming along.
