I'd choose bark - as it decomposes(very gradually), it will assist your clay soil and act like a soil improver.

I have the same problem with clay soil. Both bark and pebbles will retain moisture well. I use a woven membrane (Mypex) as well under the pebbles and the soil never dries out. Of course a membrane will hinder the growth of plants (moving sideways) but it also keeps weeds down. Pebbles do tend to compact the soil over time if you walk on it but overall, it works for me.

I agree with Bamboo bark.

I will price them both up and see how it goes,thanks all

Clay soil actually holds moisture quite well. The problem is getting water into the soil to begin with! Also, clay soil will look dry quickly because the surface turns color and cracks even while the layers just a cm down are still moist. It's only an illusion of Death Valley, not the real thing! A slope in clay soil can be a bear, because rainfall or sprinkling tends to run off instead of sinking in. Medium to large bark nuggets can help by slowing down the runoff, and by keeping the surface from drying out and becoming waterproof. Large pebbles will do the same thing, but are a little less efective at limiting evaporation. Either way, don't be stingy--you will need a 5-10 cm layer to get the best effect.