You could take a look on the Ford Focus Owners Club forums for any similar issues. I used it when I had my TDCI.

I seem to recall that a failing DMF gives rise to clanking/banging noises and clutch pedal movements when starting the car with the engine warm, so you might be able to rule that out.

Could be worth checking the operation of the EGR valve and cleaning it out as well to eliminate that issue as I understand a blocked or sticky valve can give rise to jerky running and lack of power. It's quite easy to access.

Also maybe run some decent injector cleaner through in case the issue is a partly-blocked or dirty injector? BG244 fuel system cleaner seems to get good reviews.

If the problem gets worse and the glow plug light starts flashing, it means a fault code has been written and can be read to maybe throw some light on the issue (might be worth getting the codes read anyway, then cleared and see if any re-occur, indicating a current problem).

think it may now be sorted.i took it to A Ford dealership who discovered the engine management system required updating. once they had done that and recalibrated the injectors (at a cost of £80-paid by evans halshaw) everything is now running smoothly. the average mpg has improved by 10mpg too!!