welcome to GoY, can you describe the damage to the leaves or better still post a photo of leaf and critter if you can.
Its difficult to advise with out more info.

i will have to get a photo, wont be for a couple of days though, its in my friends garden, the bug is a sap sucker & lives on the underside of the leaves, it leaves a sooty residue but i cant find this bug in any book, it's not an aphid though it is a type of fly but very small.

the sooty residue is a secondary fungal/bacterial infection, feeding on the honeydew[sap suckers 'wee']. will wait for the photo, but a genearl purpose systemic insecticide should solve the problem.

Thank you I am sorry I cant show a photo as I am new to computers and I do not have any way of putting a photo on here, I need to learn what I am doing, I do not like using chemicals but sometimes they are all we've got against these bugs,I have found a spray of antibacterial washing up liquid has seemed to help some plants fight off infection by themselves,I have not treated the Pieris with anything yet,I think I will try the antibac first and see how that goes before I get the serious treatments out,but thanks anyway I'd forgotten about the sooty residue being secondary the antibac spray will help clear that off the plants foliage and probably get rid of a lot of the flies at the same time :)