Richard -

thanks for the info - I've got it working now...

I created a blank document in photoshop with a transparent background.
Then used the magic mask tool to select the white areas in my graphic, then inverted the mask to select my object and copied it in to my new transparent document.
Saved it as a tiff, with the settings suggested - and it works brilliantly, with no need for transparent colour effects. - Object floats over background with no noticeable distortion

One of the issues I had was that I was using coreldraw (PC) to create the tiffs - and it doesn't save transparent backgrounds, although tiff format supports them... Basically it was merging the transparent with white... hence no transparency seen.

Got a bit of work to do converting all the relevant tiffs - will try your second method mentioned.

However, I can still use transparent white or tint colour effects for making simple text graphics on white backgrounds transparent

Thanks for the help

S