Cheers for your help tomse,
Here's the bottom of the pcb:
Looks like there's quite a bit of residue on the underside.
BTW my soldering skills aren't exactly great...you can see where I added in the headers.
I also put the jumper back on to DS0 - thanks for pointing that out!
I've tried flashing it several more times over the last ten mins, but still get the same message as my initial post