Well Jeyes Fluid will 'kill or cure' most things - though it can no longer be sold as a pesticide!
Save your knees and get a good hard (twiggy rather than nylon) yard broom.
If you have access to one you could try a pressure washer but long term lichens will grow unless you keep on deep cleaning to stop them. And, really, it's just part of the weathering process that gives the paving a bit of character.

Can I ask how long the paving's been down? And is it just concrete based paving, or do you know what material they're made out of (I note the sand in between rather than mortar pointing, which would point more towards a type of paviour)?

We just accept that this happens if you have natural stone rather than concrete slabs adds to the stones in my view.

Thanks for your advice..I will give the Jeyes fluid and a hard brush a try.Urbanite..We have tried the pressure washer,which didn't make much difference..,and Bamboo,they have been down 13 years, but the marks have only appeared over the past two or three years..my OH thinks they are man made,but just look like a stone circle..they are bedded on a membrane and sand,and we had just removed all the moss growing in between, them,and filled in the spaces with sand..
I suppose it does add character ,Mg..but it just looks like chewing gum in my eyes,that you see on the streets..which it definitely isnt :o),but thank you all for your help,much appreciated..

I use hyper chloride you can get 5 gall drums in the agricultural stores.A lot cheaper than these gallon path cleaners in the garden shops. I wet the yard slightly then pour a jam jar of Hyp... into a watering can full of water, pour it all over the yard and brush it in with a cane brush and leave it, do not rinse it off, let it soak in gradually It will take a few days before you see the full benefit of it. I do mine about twice a year and it certainly brightens up the place. I was brought up on a farm where we used it to wash the milking parlor and utensils up every day and our concrete always looked pristine. But I would not like to go to the hard work of doing my patio every day. but that is where I got the idea from and it keeps me happy. Some of those lichens can be a bit stubborn but it certainly stops it getting too bad.

They look more like efflorescence to me - if it is that, it should disappear on its own, gradually, over time. Quite why they've effloresced after that length of time is hard to say, but something's caused it.

I have put a few photos of the spots on my concrete (If you click onto my profile picture you will see them) but I am quite sure in our case it is caused through the pollution from the planes as the landing flight path to Cardiff Wales air port is over us and with the prevailing wind from the west I only get this problem in the front of the house where the pollution drifts against the house. The patio close to the front gets these black and white spots but as you can see in the photos gets less further away. I don't get them to the rear. I do not worry about them as it does make the boring concrete a little more interesting!!! but do treat them about twice a year to keep them in control.

My pavings get spots similar to these ... I've always presumed the discoloured areas are caused by bird droppings .. pigeons etc .. and I remove them with a wire brush .. but it is very hard work.

Thank you,for your interesting suggestions there,much appreciated.and thank you for posting photo's Telme8...I guess it could be a combination of lots of things....I don't get the spots on any other areas of paving or stone walling,which I have quite a lot of,only on these circular ones.,so I think I have discounted bird poo,Terra,as there is nothing on the areas near the bird feeders,which I would have expected.,and Efflorescence could well be another reason,thanks Bamboo..I shall try the Jeyes fluid forst,and see if it makes a difference..or I might be better getting the chalk out,and joining up the dots ! Lol..I might get an interesting picture :o)

Tel's Middle picture definitely shows the early stages of lichen growth.
Great close up pic of this fascinating lifeform at http://www.naturespot.org.uk/species/chewing-gum-lichen

You can encourage lichen to grow on stone and concrete etc (to age the pristine statues bought over the holiday weekend) by painting with a solution of yoghurt and/or manure.
I was given a terracotta gargoyle a few weeks ago - the giver thought it was a (Welsh) dragon as it had wings! I don't go in for garden ornaments but this one will have to be found a perch somewhere.

Thank you Urbanite..It does look more like more mine,in Telme's middle photo..