Power washing

I'd imagine the most cost effective way would be to hire a washer and do it yourself, if you dont know anyone from whom you could borrow one.

Ot, if you want a less immediate solution (and not really as effective but it still gets rid of that green mould), spray Algon on it on a dry day - costs about a tenner for a big bottle of it in a garden centre, takes a few days to work but it's good.

HeidiHeidi wrote: »

I'd imagine the most cost effective way would be to hire a washer and do it yourself, if you dont know anyone from whom you could borrow one.

Ot, if you want a less immediate solution (and not really as effective but it still gets rid of that green mould), spray Algon on it on a dry day - costs about a tenner for a big bottle of it in a garden centre, takes a few days to work but it's good.

Patio Magic is a better solution, as it contains 10% Benzalkonium Chloride, whereas Algon's active ingredient is Acetic Acid, so it's just an expensive way of buying vinegar.
Products like Patio Magic (Lidl occasionally sells their version quite inexpensively), take a week or two to take effect. I use it around my garden and my boat, at a dilution of about 4:1 and the effect takes about a week to show and improves over the next couple of weeks. It lasts about six months, when I need to apply it again.

If you do go down the power washing road be careful not to wash out the grout between the slabs. If you power wash you would be well advised to start using a Patio Magic-type product immediately afterwards, to maintain the cleanliness.
Some info here:
ps://forums.ybw.com/index.php?threads/patio-magic-et-al.471521/