Hi Wedge
It sounds like your pleco might have a bacterial infection caused by the Columnaris and/or Aeromonas bacteria.
This develops when conditions allow a fish to become stressed and his immune system to become weakened. These conditions include, but are not limited to overcrowding, uneaten food in the tank, low oxygen content in the water, sudden increase in water temperature, elevated nitrites, insufficient or infrequent water changes.
If you were in the US, I would recommend that you use antibiotics to treat this condition, but since you are in the UK, I will suggest you try Waterlife’s Myxazin or Interpet’s Anti-Fungus and Finrot. King British also makes an anti bacterial infection medicine that I have heard very good reports about.
Please do a good water change and tank cleaning before starting the treatment and keep you water temperature no higher than 24 degrees C., and make sure that your tank has good water circulation so that there is plenty of oxygen in the water.
Once the problem is cured, I suggest you increase the size and/or frequency of water changes to prevent its recurrence.