Need to improve soil what's best?

Anything is good -it depends on how deep your pockets are-buying in bagged products is going to cost- far better if you can get hold of some well rotted farmyard manure if that is available in your area.

How big a border it this?

I'd quite happily just turn the bark into it (if it's been down a year), use your own compost and a good sprinkle of blood, fish & bone.  

Then top dress/mulch as you usually would 

Double dig in some mushroom compost or well rotted horse muck to improve the soil. If you can't get it for free, neither are that expensive to buy. I dug in mushroom compost on a new bed last year and everything is romping away.

I like the advice cufcskim has given you thats exactly what i would do. Give it a right good turn over get in there while you can. Gosh I wish I had a new border Ive no more room my husband has warned me Im to leave what grass we have left alone! Ive already given up the grass in front garden and that I just got away with..

Your making me jealous. Our old neighbour has joked that one day he'll let me take over his garden you just never know.. I too just love plants its an addiction. I do gardens just to help out my neighbours Ive been known to pop up out of manys a border.