So many threads on fruit flies! Boring, I know!.
Just wanted to say there IS a trap that works, but it's not available in the UK as far as I can tell. I used to use it in Germany and it was a mircale: flies reduced by about 95%.
It's available on amazon.de and I ordered it to Ireland. You have to pay postage, obviously, to the UK. Here is a link: www.amazon.de/-/en/Fruit-Flies-Lebendfalle-trapango-%C2%AE/dp/B010O52076/ref=sr_1_6?crid=1870VUHHCYAU2&dchild=1&keywords=trapango+fruchtfliegenfalle&qid=1600526322&sprefix=trapango%2Caps%2C161&sr=8-6
I bought a pretty ceramic one in the UK for about £18 and it was useless: a few flies went in, but they crawled out again! This one traps them really; it isn't pretty as it's plastic, but it works.
The alternative is to vacuum them, but you have to keep the vacuum out and repeat every hour because more come.
I tried to upload a video of my trap full of flies crawling but it didn't upload and anyway, it's a bit off-putting! The photo on the amazon add is accurate, just imagine them all crawling around. Ugh.
My good deed for the day.