This is an embarrassing question.

How do you get strong specular highlights on a transparent object using the Mantra Surface shader?

Am trying to render a clear, thin plastic. No refraction. Have strong specular highlights before adding opacity settings. Once opacity is added the specular highlights fade away also.

I must be missing something ridiculously simple here.

Thanks

No shading expert here but I wouldn't use opacity for transparency. I would turn the diffuse off. Then just increase the spec and decrease the angle until the sharpness is right.

Mantra Surface has the Conserve Energy option on.
This means as you decrease the Diffuse intensity as you make your surface more transparent with the Opacity parameter, you have to increase the Specular Intensity in the Reflect Lights tab.

Read the Diffuse Intensity and Specular Intensity as a percentage of light reflected and the remainder is the light absorbed by the material.