Have had a trip to the vets, her opinion was the it was more than likely that the fish has developed a tumor. Risk, low chances of survival and cost to operate mean I don't think I can do anything if this is the case.
However she did say there was a chance that the problem was caused by constipation or a tape worm.
I've isolated the fish and am in the middle of a 5 day anti-parasite course and am buying bloodworms to deal with these possible causes.
My dilema now is whether I should reintroduce the fish to the pond or put it down (it doesn't seem particularly distressed but I don't want to put the rest of the fish at risk). If I did the latter would it be appropriate to send in a picture of a disection so that someone could tell me if there is a possible risk to the rest of my fish? The garden centre fish store have recommended that I dose the main pond with anti-parasite chemicals, I'm not convinced, preferring a natural environment.
Normally feed twice a day, mixture of pellets & food sticks (assorted fish mix) from local store. Whilst on holiday I added a couple of food blocks, one paste the other plaster. Also the pond is now well established and I'm sure there is plenty of natural food amongst the plants.
I've had a pond (2000 Litres) for about 5 years and have about 20 assorted fish up to a foot in length. One of my 3 Golden Orfes has developed a problem. He started out by resting with his head down amongst the plants (back in the spring). I popped into a local aquatic centre and was told it was probably a frozen swim bladder problem and just to keep an eye on him. He has slowly recovered and has become more active unfortunately on returning from holiday I've found he now looks very bloated please see attached photo.