Ants

The simplest solution for ants kills all of them effectively but does not use pesticide.  You will need laundry borax, sugar, and some sort of disposable small plastic container with a lid.

(Important: borax is not a pesticide but is toxic, which is why this works. Don't use in any situation where a pet or child might ingest the bait)

Take your small container with lid, around the lip of the container, with scissors, clip holes that are about a half cm, do this in 4 or so locations.  These allow ants into the trap.

Place the lid up side down.

Mix one part laundry borax with 3 parts sugar.  You can make a quantity of this, but store in a container clearly labeled with something like "borax ant killer".

Take a half teaspoon of the mix, place it in the center of the up side down lid. Moisten the mix with just a few drops of water (enough to just wet the mix).
Put the container (with holes clipped around the edge) up side down on the lid. Place this with the lid down any place you see ants walking.

You will initially see more ants drawn to the bait, and think something has gone wrong. After a few hours, ants will decrease. After one or two days, no more ants.

When the ants are gone, wash off the trap, store with your borax mix for the next time.

Never had ants in the greenhouse.
But always clean mine early in the year using a small bit of washing up liquid and water.
That clears any bugs or their trails they have left.
Wondering if something there is attracting them, like dropped fruit or veg. Is is food that attracts the Ants towards your house.
Ants do lay trails, so once found all will follow the route to the food source.

If I get them on paths, near any buildings I tend to watch the trail they all follow.
Make up a solution using vinegar and water and spray that on their trail. It does not kill them but deters them from that trail.
You can also do the same with a citrus solution using Oranges, lemons or cucumber rind, but I would not like to have that in the greenhouse during growing season as the citrus may attract slugs and snails.

The most effective method of killing ants that I have used over the years is making small piles of Cornmeal.
So work out the trails, see where they are coming up from and place small piles of cornmeal. They will walk all over it, so keep on it and keep it confined. They consume/carry it and take it back to their nest, but they cannot easily digest it and it kills them and others that they feed/take from the delivery to the nest. It takes a few days and you may see a few more ants appear for a short while, but it works.

Nippon drops Mrs Bean, Nippon drops on a washer or old penny!

Cheers,  Tony.

My apologies / mistake, then.

I am curious why it was banned?  It is still available in the USA.  I guess I can look up the "why".

No apologies required, just wanted to let others know, that what they can buy now wouldn't work 
I've no idea why it's banned, because it's only seems to be bad for you in the same way that ordinary salt is - in other words you'd need to ingest tons of it before you did any damage  Bureaucracy gone mad..

Thank you Kletiwallah, sorry but what are Nippon drops x

Liquid ant killer, use a few drops at a time...
https://www.webbsdirect.co.uk/nippon-ant-killer-liquid-25g/

Sugar plus the pesticide Spinosad (considered "organic" safe in the USA, not sure about EU / UK).