I've just come back up the M1 at a steady 70 mph.(yes really!)and the engine was misfiring beyond belief. Kick back hard when revved with no load later. I'm discribing it as a misfire but it was a bit like the brakes coming on/clough splutter type of hitting a wall momentarily. All this at around 3k revs. I've just done 2k miles round France without a hic and returned about 31 mpg so I'm surprised at the sudden change. I'm concerned about expensive repairs. Any ideas? Head gasket? Turbo? There is a slight loss of coolant (small puddle on floor under rad)and runs a little hot in town.
The car's a LP turbo CSE,K reg with 104K on the clock.

I'd check the coolant leak quickly and why its running hot. Is there any smoke out of the back when it 'misfires', how long does misfire last for, is it only at steady speed or under acceleration? My first reaction is the DI unit but that would only give a misfire rather than a hitting the brakes feeling (but the latter is a very subjective description). Try cleaning contacts between DI (red black holding coils above spark plugs) and its lead, look for any signs of oil around the coils, if they are leaking the unit is shot - new one is approx £125.