Get it into a tank of clean, cool water ASAP. A quick hospital tank could be just a tub large enough to allow the fish to move in a tight circle.

Add an airstone and a pinch of any good broad spectrium antibiotic. Do not feed it. Just do water changes and if it's a large fish in a small container, change several times daily.

If a carp was strong and healthy before an injury and the injury is not too traumatic, if allowed to rest in super clean water to ward off infection, it may pull through.

Meanwhile, do something about that net, as it may have been the cause of the injury.

Quibloid. the first thing you have to do is remove the koi from the pond into a 'hospital tank' where you can attend to the cuts.

You will have to use an antibiotic treatment to stop infection in the wounds.
Your local pond specialist store or veterinarian can advise you on that treatment.

Since you live in the UK I'm not sure what did the damage to your fish?
Here in North America, that kind of wound can be caused by a raccoon, but you do not have them over there.
Maybe a fox, heron or even a cat got at your fish.

There is a UK pond forum [still with Garden Web] where you can obtain information that could be more relevent to your situation.
Click the link below to see the UK pond forum.

Good luck with your fish and please let us know how you make out with it.
I have posted your question over onto the UK forum.
Maybe some one over there will give you some more information about treatment. Use the link below to go there.
"Horton"

Here is a link that might be useful: UK Pond Forum.

Thank you birdwidow and horton for your advice. I will do as you say and hopefully it's not to late. Will let you know either way. Kind regards Julie.

Julie, you are welcome, I hope everything will turn out okay. I only wish I could be of more practical help to you.

I e-mailed "Scotty" a member of the UK pond forum and asked her to read your post, that I transferred over there.
I hope she will contact you soon, as she is very knowledgable about ponds and fish etc.
Good luck in the meantime with your fish.
"Horton"