I bought an 03 Focus 1.6 about 18 months ago without a service history. We tend to run old bangers into the ground and take pot-luck with neglecting the servicing, we usually get away with no expensive bills :-).
I'm about to book it in for an MoT and get the oil changed while it's there. I've heard about non-approved plugs and leads damaging the coil pack, but is it also important to change them regularly even if the car's running OK? How much is it likely to cost?