\def\myfilename#1{{\spaceskip.25em plus 1em\relax\tttf\normallanguage\zerocount#1}}

\myfilename{Transfer functions for PID and KF controllers.xmcd} \myfilename{Transfer functions for PID and KF controllers.xmcd} \myfilename{Transfer functions for PID and KF controllers.xmcd} \myfilename{Transfer functions for PID and KF controllers.xmcd}

That works brilliantly (I've no idea why through). Thanks Hans