Hi Neil
Would seriously recommend the Jitsie or Apico range of handlebars. The come in a variety of sizes 4.5", 5.0" and 5.5" plus a choice a colours. I sell these handlebars here in Australia and the riders love them, well priced plus very hard to bend or break them. Not sure where to get them in the '"ole dart" but I am sure someone should be able to point you the right direction.
Keep ya feet
Kal
KD MotoTrials Australia