we often find them in the fruit bowl if fruit has been in for a week or so. I like very ripe bananas [almost black skinned] and that is often their source.
are your tomatoes in the house? The flies are attracted to ethylene gas which is naturally produced as fruits ripen.

I was taught by a florist over 50 yrs ago never put flowers and bowls of fruit near each other as the gas makes the flowers fade faster.

Thanks as per, Seaburngirl! You are showing your age now, just like me!
Come to think of it, someone gave me a couple of plantains and the skins had been thrown in the rubbish bin. However, the flies went for the HP Sauce, so I forgot the skins. I emptied the rubbish and the flies were still on the sauce!
The tomatoes are still outside. I do have a couple of very green ones on the sill but the flies are nowhere near those.
I still think it a bit early for fruit flies though. The sauce has been thrown away. There are still a few hovering around and seem to come out of nowhere.

Are they fruit flies, or are they fungus gnats.

with central heating and food coming in from all over the world fruit flies are always able to be present no matter what time of year.

Thanks for joining in, Tugbrethil. They are definitely fruit flies as I have had them before when the tomatoes ripen. I have not heard of fungus gnats but sound as though they blight your crops from what I have just read.
You have now cheered me up even more, Seaburngirl! I have never had them at other times of the year.
I don't like killing animals but draw the line with these little blighters by putting a little wine in a bottle with a funnel. However, I have been reading about how to deter them in the first place.
Apparently, they don't like - presumably the smell of - cloves, basil and lemongrass. I have put a few cloves and some basil out. I have just successfully prepared some strawberries but maybe I was lucky. I also have a lemongrass candle so will give that a try.

I only asked because fungus gnats and fruit flies look superficially similar. Of course, while gnats get into everything--what a nuisance!--they don't actually eat the fruit.

Thanks again, Tugbrethil! I was just thinking, Seaburngirl, now and again I buy the odd banana, at any time of the year. I don't recall any fruit flies when I put the skins in the bin. Unfortunately, these things can't talk but it would be useful to know why I had them with the plantain skins.