I have replaced the spark plugs in my 99 focus (i.6 zetec) after 1 mile the engine management light has come on. I have put back in the old plugs the light remains on car running fine

 

Unlikely that new plugs would have caused the light to come on. In order to make the light go away you will need to disconnect the negative lead of the battery for about 10 mins to reset it.

Did you put the HT leads back on properly? Was it misfiring at all?

 

Running fine