Hi cleaning your stone balustrade should be quiet easy,I take it it has general staining maybe moss where it's in the shade possible rust stains,there are some serious cleaning agents that can be bought from swimming pool materials outlets but before you splash out(could not resist that one) try this buy a 3 or 4 inch brush a cheap one,buy some Oxiclean it's advertised on the TV I saw some cheap at Lidl's supermarket brush that on then with a scrubbing brush work it in, and leave for about 20 mins then get your pressure washer and on 1/4 power wash off ,this may well clean it up ,if it doe's not get a bottle of thick bleach pour it into a bucket and add same amount of water plus a small bottle of babys liquid soap mix all together,make sure you have plastic gloves ,googles and old clothes on then do the same cleaning routine as before,only thing is when you wash it off if you have plants near it you will have to put plastic sheet over them as this mixture will kill them.