I would change the leads right away, what plugs are you using, not BOSCH I hope, if so bin them and fit ford/NGK ones in there the bosch track massively in the zetec.
The normal zetec issue is with the coilpacks breaking down, but you'd feel this easy enough.
As a rough guide my dads 2L auto gets on average 38mpg, driven gently.
If not done so change the fuel filter, and clean the MAF, use carb cleaner, do NOT under any circumstances poke the hot wires, just spray liberally with carb cleaner and shake dry.
The pull the violet coloured 3 amp fuse in the fuse box next to the battery for 1 minute, make sure the maf is plugged in, this(pulling fuse) will delete the fine trim settings stored in the ecu, and it will relearn the fine settings, just drive normally it does the rest, it can take a few miles(about 50) in that time it may be a bit lumpy.
With the age of the HT leads though I'd be amazed if you don't see a big difference right away running wise.
Get yourself some silicone grease, and smear the inside end of the HT lead plug end, zetecs can be a bit fussy and track easy if there is the slightest tear in the plug boot, for issued a TIS about this as well.
If you use a K&N filter the maf needs cleaning at least every 6 months, the oil impregnated filter's oil gets on the hot wire and of course being hot it sticks, and throws the reading out.
tabetha