How can I fix misfire and loud tappet noise on R reg Ford Fiesta 1.3 Encore?

There is a loud tappet noise from the engine - on investigation it appears that No2 cylinder Inlet tappet has no further adjustment left on it - and the push rod sits lower than the others (and they are all the same length) - consequently when the engine is running the valve is not depressed by the rocker. I have adjusted this tappet on a number of occassions, it did run Ok for a day or two but the noise and misfire returned - I have now run out of further tappet adjustment (ie the tappet is screwed fully down) - Where do I go from here please?