My 2003 Comfort Estate 1.4 16v loses a small amount of coolant and if I use the max demist setting I get a slight oily film on the windscreen.

 

I'm guessing the two are related, a leak in the heater matrix ?

 

It's not a major problem, more an annoyance, is it worth putting in a dose of radiator seal before things get any worse ?

Radiator seal might fix the matrix leak, for now, so maybe worth a try.  Does the oily film look pinkish if you wipe it away with a white tissue?  I can't think what else it would be other than coolant, but a colour match would be good evidence.