The spark plugs will need replacing every 12,000 miles. At the 6000 mile service you should remove them, clean and re-gap them. Treat yourself to a decent spark plug spanner. The Capri like some other Fords uses 18mm plugs which are unusually large. I bought a spark plug socket for my socket set which works well. To remove them, carefully pull off the HT leads as close to the plug as possible, noting which one goes where (they should be 4 different lengths). The plugs can be tricky to remove; take great care that the spanner is not twisted or you will break the insulator. The tip of the plug should be brown and powdery in appearance, not glazed, sooty or oily. The Haynes manual has the pictures for you to compare with. Give each plug a gentle clean and then using a feeler gauge, re-gap each electrode to 0.8mm using a feeler gauge. The gauge should just squeeze in the gap when correctly set. If they are new plugs, don't just assume that they are supplied with the correct gap. Wipe any debris away from the threaded hole before replacing. A light spray of WD-40 works wonders when doing this. Tighten the spark plugs to 20 pounds foot with a torque wrench. Make sure the HT leads are replaced correctly. They should fit over the spark plug with a positive 'clunk'. Can I suggest that you replace the spark plugs at the end of the service as this will make some of the other jobs easier?
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The HT Leads
I would advise replacing the HT leads every 50,000 miles to avoid problems. The carbon electrodes break down over time causing a weaker spark and the silicone insulation also ages in the intense heat under the bonnet. This may eventually allow moisture to penetrate and cause all sorts of problems. If you are an electronics whiz you could try measuring the resistance of the HT leads; a new set are in the region of 2000 ohms depending on their length.
Every 6000 miles, I would recommend cleaning the leads with rag damped in turpentine or lighter fuel to make sure all traces of oil and dirt are removed.