Can I use bark chippings as a mulch in my planted wall?

Yes you can use bark, but like the pea shingle it will get blown about. 20 ml. pebbles are the best mulch as nothing comes off them and they don't absorb any chemicals etc.
Keep the top of your beds open and break the capillary action.

Thanks DoctorB - not sure what you mean about breaking the capillary action though.

You can get round the problem of nitrogen depletion by applying Growmore every six weeks; the depletion is caused by the bioactivity required to break down the bark chips, including nitrogen, so the plant doesn't get any while its going on. It's a bit of a bore with growmore though - might be easier to water on Phostrogen, at least you don't have to move the bark chips out of the way every time.

If the top of your flower bed is hard, moisture will evaporate quickly, i.e. small holes. If dug over you will make larger holes i.e. less evaporation of moisture. In a short while the rain and the weather will make the holes smaller. 20ml. pebbles will not break down and the cracks between them (holes) will stay open all the time. If you lift them even on a dry day there is moisture underneath, which plants need all the time. You can walk on the pebbles at any time of year pulling out weeds etc. You can feed through it giving the whole area a natural environment. Hope this has explained it for you.

Make sure soils wet too before you put bark on too.

Thank you everyone for your contribution and explanations. I didn't manage to get any mulch at all today - took daughter to park instead! But will take everything on board before deciding which to go for. Thanks again!