My Rouse Cosworth has a very slight misfire on tickover, its fine once its going, any idea what it could be?

 

Could be loads of things, cloggy injector with bad pattern, to dicky spark plug, and anything else in between.
Try and pin it down to 1 cylinder by pulling each plug lead in turn, don't electrocute yourself in the process, do this first as it makes the fault finding easier.
tabetha