Right - the tracing paper arrived first thing and a test shows it gives much better blacks than the transparencies I was using (dunno the brand) with the laser.
In the search for perfection, the printer allows a choice of paper - thin thick coloured etc. Which would give the densest black? I've got it set to 'darkest'.
Anyone hazard a guess as to exposure time versus transparency? It's such a fiddle doing test runs.