Ford Focus Mk 2 - Misfire after core plug replacement

Hi all,

I have a 2005 Mk 2 Ford Focus (1.6 Ghia). A month or so ago it started idling rough and misfiring badly, the engine light initially flashed and then stayed on. Reading around the problem led me to check the core plugs, one of which was shot, and the coolant level was low. Took it to a garage and they replaced the core plugs and spark plugs.

Following the work it's been running better, but does still have a bit of a misfire, generally when accelerating (especially uphill) in 3rd, 4th or 5th. The engine light hasn't come on and it idles fine.

I removed the rocker cover and there seemed to be a little bit of dampness in there. Spark plugs 1 and 4 were both slightly damp. The coolant level hasn't dropped. The garage appears to have used sealant around the plugs, and they look OK. The HT leads don't look corroded, and the washer jets (which I've read can also cause this issue) are sealed.

So I'm wondering what is causing the misfire now? If the core plugs are not quite properly sealed, then wouldn't it take a while for enough coolant leak to a point where it was reaching the plugs? And wouldn't this then show by a drop in the coolant level? Could it be that there is still moisture in the engine somewhere from the original leak?

I'm at a loss so any thoughts greatly appreciated. Thanks in advance!