Ford focus mis firing nightmare

colino

The original repair if I recall correctly cost you £72. No one took off the head or did any serious repairs for that.
Core plugs either fit or they don't, either the properly sealed screw in ones or the dish tap-in ones once properly fitted, last for years. If it's leaking "again" from the same core plug, it wasn't replaced.
The two faults of weeping core plugs and dripping washer jets seem pretty common on Foci. Both will pool water around the spark plug and can cause a misfire. That misfire will be held in the ecu as (a minimum) P0300 (random misfire) and then P0301. 0302. 0303 or 0304, depending on the specific cylinder registering a problem
All of those codes however can be thrown by a whole host of things from dirty air filter, blocked fuel filter and another dozen possibilities.
What you need is a recommended, local, friendly garage to look at it. They are very common cars with very common faults so someone who can't diagnose and fix pretty swiftly needs to be struck off your Christmas card list.

catoutthebag

Last week, the car was back to square one. Really juddering, clanking & lacking power. Quick look under bonnet showed coolant really really low - way below minimum

Had it towed to a reputable garage last week - they'll be examining today. The guy told me that ford core plugs have a weak spot on them causing them to leak.

But once confirmed, ill have it sorted hopefully - and will also writing to trading standards about the previous garage