they are a nuisance but don't do any real harm. Their larva feed on the plant material.

try turning the contents in the bin so that the conditions change and this may deter them.

They're generally attracted to the rotting stuff so if you put a layer of the bottom compost over the newly added bits that helps. You can also use diatomaceous earth which is an ultra-fine grit. It gets into the insects' body joints & they die. It's okay for worms though. Last time I looked it was about £15/kilo but you can buy smaller amounts

but many of the insects are beneficial so why would you want to destroy the beneficial ones?

There are hundreds of invertebrates all working in the compost bin. there are herbivores and predators many less than 3mm long.

I find many of the flies in mine are in fact types of fruitfly feeding on the rotting materials.

When I put shredded paper on top they seem to disappear for a while.

I find a spadeful of soil over the top sorts it for a while.

I don't really understand why they are a problem Alanturk. They seem a natural part of the composting process to me and so I accept them as much as the worms, slugs and woodlice.
As SBG says they are probably beneficial in some way.

Thank you everyone for your comments.

I can fully appreciate that the flies are part of nature and that are almost certainly doing a great job in my bin. For me, the problem is that when I open the lid they come out in a great cloud around me, which I find quite unpleasant. Maybe some would enjoy that, but I have to say, it's not for me! They often land on my skin and my clothes and I can't help wondering what bacteria they may be carrying.

Also, I may be inadvertently carrying them indoors, when they might take up residence on my houseplants. Again, some might find that acceptable, but I'm afraid I don't.

Anyway, I'll try some of the suggestions, for which, again thank you very much.

Talking of fruit flies, if you leave something (recently Lemons) they will be covered with little flies. Now are they in the fruit already or how do they get in to get to the fruit?

I think they've got a really good sense of smell Honeysuckle, they're attracted to carbon dioxide which is why they fly into your face when disturbed.
You can use it against them by half filling a jam jar with water, adding a couple of teaspoons of sugar or honey & a drip of washing up liquid. Give it a swirl to mix then sprinkle a good pinch of yeast in, any type will do. As the yeast ferments it gives off CO2 which lures them to their doom

there are probably some fly pupae in the crevices in the skin. and when they emerge they fly freely.

Alan try leaving the lid off for a little while and then the flies will either disperse or get eaten by birds.
I don't like them flying in my face either, so you are not alone.

When you say you add kitchen waste that should not include cooked food (sure you know this but just in case anyone reading this doesn't...)