Can any of you gardeners help?

Sounds more like lichen which you can get cleaner for.

It’s probably efflorescence- which is a leeching of the natural salts/minerals in the stone/slate. We have it too as our path is newly laid and we haven’t got round to sealing it yet.
If you Google it you’ll find ways of treating it without damaging the stone.

Brush and clean the patio flags and seal them. I used Ultimate Sandstone Sealer when I had my drive and patio laid last autumn and was advised to do it bi annually.

We used our power hose (not as fierce as commercial ones!) and it did the trick.

Thank you for your replies. We have brushed it with a stiff broom, and then power washed it, and also at other times used patio cleaner, but nothing shifts them. It isn’t powdery.

Can you not go back to the company which laid the patio and ask them for advice?

NfkDumpling hoping to do this when we find the paperwork with the company name which neither of us can remember!

Found this, any natural stone is susceptible to this kind of reaction.

How to remove efflorescence from natural stone and brick.
Stonedoctors are often asked for advice on how to remove white residue from the natural stone or brick. The white residue is in fact is called efflorescence, or in other words the natural salts and minerals found in natural stone and brick.
This is a very common problem.
What happens is that natural salts in the material are carried to the surface and deposited by water penetrating into the material and moving through the microscopic pores in the brick / stone. Once the water evaporates from the surface the salts are left and the sun then solidifies the salt deposit on the surface of the material.
However more often that not the salts have hardened and an acidic cleaner is needed like our Stontex Cement Away to break down the salt deposit and remove it from the surface. However some materials are acid sensitive. A Hannafin cleaning product called “Eff – Eraza” can also be used on the stone wall or paving. This is a safe acid designed to remove these stains from acid sensitive materials.
If you are not sure how your stone or brick reacts to acidic cleaners the best approach is to test a small area first. If you are happy with the results you then can apply the product to the rest of the area.

Thank you for your help, much appreciated