Pressure washer, although currently, it will create more unnecessary water in the soil as it runs off...

I agree water. and if no pressure washer then a very stiff brush and lots of elbow grease. No quick easily solution that wont have an impact on the soil etc.

I use a Pressure Washer, the slabs come up a treat, it seems to take forever but worth it in the end Phew!
Oh! Best wear wellies too LOL as trousers will get a soaking :o)

Thanks everyone.

I use Algon which is an organic based cleaner that you dilute and either put on with a watering can or sprayer - the sprayer being more economical. It is safe to use near plants and animals -though can scorch the grass a little if you spill a lot on your lawn.

It can be bought from most garden centres. Because I needed a lot at one time I googled the manufacturer and had it delivered to my home. Each 2.5 litre can costs around £6/7 depending on supplier and once diluted will cover 60 sq mtrs.

I do my patio area most years, it takes away the algae and grime but takes a few weeks to show a clean effect though the slippyness is removed within a week.

Try it, it is brilliant and needs no scrubbing. Ask at your local garden centre as they have this same substance often under a different name and a little more expensive.

Thanks Janpled