I very much doubt it has anything to do with the petrol more than likely a disintergrating cambelt pully.

Do the old "screwdriver to the ear" trick,amazing how it amplifies rattles&knocks.Used it dozens of times to locate problems on cars,bikes,tractors etc.

Put the plastic end to your ear whilst placing the flat blade to whatever you want to listen to,preferably without getting it caught up in any pulleys&belts.Not recommended for use near ignition systems&doesn`t work very well for tracing suspension orientated clunks!One day I will invest in a stethoscope....but enough about my personal life!

In the dark under the bonnet,check for arcing from the plug leads/cap.Jiggle leads around whilst engine`s running,looking out for blue sparks.Had similar problem and mine was a faulty plug lead on No2 cyl.

Fuel filter on my 2000 zetec is in front of rear axle,n/s.Original,120,000 miles.Found it when replacing the brake pipes,removed metal clip with pliers and double cable-tied back in position.

Assuming you haven`t picked up bad fuel from contaminated forecourt tanks(It does happen,this year one locally had such a problem after drain overflowed and several vehicles ground to a halt after filling up)then replace your fuel filter(£4-£9) check for gunge in the line, bung some fuel additive(£10-£15)in your tank.A mate took his `04 Focus td in the other week for MOT and it failed on emission's so the tester poured some "Forte" in his tank,told him to drive around the block,stuck the probe up the exhaust and it flew through.Cleans petrol injections too.

If tank is contaminated you will have to either remove it or pump contents out before doing the above.Having drained many tanks I honestly don`t know how some vehicles manage to run.Probably the norm in some countries.

Thank you for your reply. I will try the screwdriver to see if there are any knocks. I did not suspect a camshaft pulley. The poor running happened after using the second tank of French petrol so it would be a startling co incidence if it was the camshaft pulley. I don't think that it was contaminated petrol but rather that the new petrol in France is incompatible with cars pre 2000. There have been many reports recently of cars being damaged. The new petrol is 10% ethanol which apparently is corrosive to metal. I will search for the filter.