Look here.
https://www.rhs.org.uk/advice/profile?pid=804
They do not nibble leaves by the way so something else is eating your Avocado leaf.

that's the link I was going to give Owd.

if you can let the compost dry out a bit that will help kill off the larva that are in the soil.

Thank you for the link.

I have tried to protect some of my cuttings by covering the soil but the gnats seem to be able to find their way into this . So spraying with insecticide doesn't work because it is the larvae beneath the soil that does the damage.?
I can't imagine what must have been nibbling the avocado leaf, the nibble marks are far too minute for a slug or a vine weevil

Thank you both. I will try to let the compost dry out. How about base watering eventually?

An inch deep of grit on the soil surface if there's room will help. Also the old-fashioned fly strips or yellow sticky traps are effective if unattractive.
I've had them recently from the compost bucket in the kitchen & used a mix of cider vinegar, honey, a couple of drips of washing up liquid then some water swirled to mix. I just used a glass jar with a funnel-shaped piece of paper in the top to make escape difficult

Thank you Darren8

I am curious to know why you added the honey? Does its odour attract the adults?

You thought of fruit flies but I doubt it is these as they are attracted to sweet smelling things and tend to loiter in the kitchen area.
Just for the record, spraying fruit flies does no good. The best thing to do is to save a little wine in a bottle or drop in some jam or similar. Then put a funnel on top. The flies will eventually find their way inside but cannot get out. Leave the bottle as it is. Sometimes these blasted things can hang around for quite a while during the harvesting season. As long as you keep the bottle there, they are not likely to go elsewhere.

Wonderful !

So it is the same idea as a method for catching wasps?
My conservatory backs on to the kitchen with just a glass dividing door which a lot of the time is open and easy for these little beasts to infiltrate the rest of the house.There is fruit in the kitchen as well as other inviting stuff, so they may well be fruit flies. How can I tell the difference between these and fungus gnats ? Do they both hang around all year?

Thank you very much.

The fungus gnats would be the ones showing an interest in your plant soil.
I just added those ingredients as I was being driven mad by the darn things & that's what I had in the cupboard but it was very effective. I'm sure stale wine would work too, just add a drip of washing up liquid so that they sink & drown quicker.
Don't forget they have a very short lifespan

I've found beneficial nematodes and Bacillus thuringiensis israelensis to be effective in killing the larvae. Also, be sure to pour bleach or boiling water down your sink and shower drains to kill the ones that colonized there.

Firstly, you need to eradicate the source, what is attracting the flies in the first place. They will show you, just follow them. Those flies are drawn to rotting food someplace or did somebody poop behind the sofa (a pet). The fungus too. Both are there for the same reason. It's not the potting soil.

I agree with above against using the canned bug spray. It's expensive and leaves a nasty oily film on everything. That's a slip & fall accident waiting to happen. The windows are a nightmare to clean too. Go with the sticky fly paper strips you can hang up and discard. They are cheap & they work.

Secondly, I would submerge each potted plant for 5 minutes in a hydrogen peroxide solution then rinse with plain water, ½ cup of food grade 3% HP to a 5 gallon bucket of water. It's about 1 dollar for a bottle at the supermarket. Do a double dunk if it was really bad. HP will kill everything, yet won't harm the plants. Do it outside so you hose down the foliage to get rid of things in there. It's like giving your plants a spa treatment. They'll look nice and refreshed afterwards.