Hi Nigel,
I'm sorry, but for now HDRWT is one of the tools that is not completely "previewable". When trying to preview the effect of HDRWT, use a preview that includes as much information as possible: covering the whole dynamic range, and areas with small and large structures.
I have some ideas to make this process previewable, but it's not implemented yet.
Best regards,
Vicent.