Sorry can't help but wonder why you want the patio to look pristine. Ours gets swept, occasionally, make that VERY occasionally, but that is in keeping with our garden.

Ah but you aren't called Posh Moon Grower! I can see that some patios might look better pristine if that's the image of your garden. Have you thought of bringing in the professionals? We had our extremely grotty one cleaned last year and were amazed to discover the true colour. They got all the lichen off it, which I don't think a domestic pressure washer would do. Scrubbing by hand certainly didn't.

if its cement try a strong water bleach solution soaking it in then pressure wash it .

Might end up removing the surface of the slabs though NP and then they would end up looking even more muchy!

Also there could be a danger of run off onto wanted plants - depends on where the patio is sited. Some commercial patio cleaners are largely bleach.

I know what you mean, we suffer the same problem and I would love to get my indian stone looking as good as it did when first laid. This company reckon they have a product that can do the job, but it`s expensive, I was tempted to give it a go but would need too much for my area:
www.patioblackspotremoval.com

Bleach won't harm the surface of the slabs MG. I had the same problem and brushed in a strong solution mix of bleach, left it for a couple of hours and then hosed it down. This helped, but it still didn't get rid of it completely so I ended up using a brass bristled brush, but the stains were still slightly visible.

So, I think that if you want to get rid of it completely, then the only solution is to either get in professional cleaners or get it sand blasted.

your right Myron as slabs are coloured in the liquid form of cement as aposed too painted . also if you wash the bleach of well and the serounding plants etc its bio degradable and reasonably save .

Bleach is not safe to use around plants, even diluted, its toxic to all life, including microorganisms in the soil.

You haven't said what your patio is comprised off, I mean what kind of stone - many types of stonework do get stained or marked by all sorts of things, not least dropped fertiliser granules or weedkiller granules. It's likely a professional company may be able to advise as to whether the marks are permanent staining or can be removed.

well I got to say my cherry tree is thriving on dog wee water and bleach and it is biodegradable unlike many things . as said though do watch your plants .

AsBamboo has already stated bleach is toxic to all life. If you have a septic tank do not use bleach in your loo!

wow and I clean fish tanks out with it .

youl find that jeyes fluid encourages algi too .