I had similar trouble. 25 to the gallon and lumpy running. This turned out to be two faults already mentioned. The inlet manifold gasket was leaking. This gave lumpy running. This is not an easy job, I'm afraid I chickened out and let the garage boys do this one. The fuel consumption increase was caused by the duff lambda sensor. This was shown up by emissions test failure. Incidentally neither the lambda indicator light nor the diagnostics showed this fault so beware!
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