[Steve Tinker]

The reason I advised Toplad to inspect the insulation of his coil packs is because he lives in the north of England where the atrocious weather patterns can quickly turn a small crack into a misfire.
When the insulation is compromised, it doesn't take much moisture to cause all or some the LV or HV to track to the chassis before getting to the spark plug. The voltage leakage may not even be enough to cause the engine to miss, but enough to lower the spark density giving poor fuel economy which the OP complained of.