concrete paving slab black marks - is this lichen?

Yes, thats Lichen, get some concrete cleaner from your nearest farmer's co-up and follow the directions

Can't see why you'd make a massive effort to clean those bits of concrete, dirty they look more like natural stone, clean they'll look like exactly what they are concrete.

10/10 for the photos btw.

Sorry OP its down to personal taste, my preference would be for the aged look rather than the clean concrete look.

Don't get me wrong I don't think all concrete should be left dirty looking. The big problem with a lot of modern architecture is the ugly look of dirty concrete but often in a garden I think an aged look is often better.

Perhaps I'd have a different view of your path if it was all one colour instead of being random. The randomness for me works better when the blocks look a bit dirtier and the colours meld together.

btw I'm not trying to offend you its just my personal taste which comes out a little too strongly sometimes because I think as a nation we have a bit of a fetish for cleaning things with a pressure washer.

Sodium Hypochlorite is what I've seen most exterior cleaning companies use. I'd rather the clean look on your driveway too, it does brighten up the place. Available in co-ops and the like, in 25litre black drums.

Have a gander at this video, he covers it fairly extensively i think.

Having said that, I think there's a place for the weathered look in a lot of gardens, but some lichen looks better than others imo.

You can get that alright. Goes by Lancelot Green Clean. There's two mistral sites, one for trade and one for is regular folk.

That said I've ordered from both sites no bother.

There's always the nuclear option. 50% DDAC from:

https://www.benzsoftwash.com/collections/all-benz-softwash-chemical-products/products/benz-bio-cleanze-ddac-softwash-biocide

You just dilute it down more than the mistral stuff though. I usually aim for 1.5-2% DDAC.

I moved into a house last year and loads of this all over the place. Was recommended to get Stontex Black spot remover. Think I diluted 1:1 and within an hour, it was all gone. I used a pressure sprayer to apply and power washer to hose away. I’d say I could’ve diluted more. Got it in Tile Merchant in Dublin. They also have option to deliver

some of those cleaning fluids don't even need to be power washed after, it works away itself.

Power washing isn't good for concrete, particularily the older concretes