Water in spark plug wells, Ford Fiesta 05

A couple of weeks ago my car started to run quite rough & the engine light came on so I took it to garage & identified that it was misfiring. The next steps were as follows:
- examined coil pack it was fine
- checked spark plug wells and there was liquid there and the freeze plugs had corroded so there were holes in some of them.
- freeze plugs replaced but problem persists.
- checked plug wells again last night and some moisture around plug thread and some around wells.

I've read that it is possible that the windscreen washer leaks in some models but I find this unlikely as there's no evidence of leaking there and I imagine it would have to leak quite a bit before it could migrate into the spark plug wells. I did have a problem a few monbths back of my coolant running out so maybe there's a leak there? If so what should I do to identify this?