I'd say at least three feet deep for goldfish, four would be better. The pond isn't really going to be big enough to keep koi or orfe properly.
Five by eight feet gives you about 250 imperial gallons per foot of depth (assuming a flat bottom). Figure 30-50 gallons for each adult goldfish, and you won't be overstocked.
A marginal shelf is a good place for plants, but it's also a good place for herons to stand and fish. If you don't want to net the pond or rig other heron deterrents, you should consider making the minimum depth three feet.
Hi Adey. Your fish are hanging out under the waterfall because there isn't enough oxygen in the water. Get an air pump and put the diffuser or stone in the center of the pond. The bubbles won't actually add that much oxygen themselves. Most of the air just goes back to the atmosphere. But the turbulence where the bubbles leave the water works in a similar way to your waterfall, and most importantly, the rising bubbles will create a "doughnut" current that will draw oxygen depleted water to the surface and return oxygen enriched water to the depths.
Removing debris or sludge will also help, as the bacteria responsible for decomposition compete with the fish for O2.
Edit: Don't remove too much sludge at once, though. It holds toxic gasses that can kill your fish.
Just a quick query i do not have any blanket weed just a thick furry covering on the sides of the pond (the full depth and bottom) is this the beginning of blanket weed or is this just general algae growth i will always get?
i do have the odd bit of stuff that looks like hair so again is the the beginning or blanket weed?
If it looks like hair or filter floss, it's probably blanket weed.
I am due to get a new UV bulb in a month or so as well, do think it would be worth getting one and seeing if this may make a difference?
The UV won't do anything for attached algae except eliminate the competition. It only kills cells that actually go through the clarifier. If your water isn't green, save your money and replace the bulb on schedule. The UV will kill some blanketweed spores, but you could completely sterilize the pond, and if conditions were right for algae, it would colonize your pond from the air very quickly.
How is your current setup different from the trouble-free pond you had before?
I put a few rocks in a five gallon bucket for ballast and then gently pull the hair algae to the bucket hand over hand. The algae sticks to itself fairly well, and hence it's possible to get most of the algae in a fairly short time.
Fredrick, what electronic device prevents or kills attached algae?
Re: Koi have been attacking up the Fancy Goldfish!
If you've kept fancies outdoors over the winter before, you probably know what you're doing. They're more delicate than the fish-shaped goldfish, and all they can do when a heron comes calling is wallow around like hors d'oeuvres with fins. But if the other fish can't get to the fancies, they can't hurt them or steal their food.
Not knowing what you have, or even what species of floating weed are allowed in the UK, it's hard to say. But koi and grass carp both love some types of floating weed. If it gradually disapperas, and the remaining weed stays green, then the fish are probably eating it. That's really not a bad thing. If it's dying in the winter, though, you should pull it out in late fall, as you don't want it rotting in the pond.
Aeration is very useful too. The bubbles don't add all that much oxygen directly, but the rising air column keeps bringing oxygen depleted water to the surface, and that will make a huge difference.