We regularly have to take the sumps off to clear the oil strainers of carbon. Quick tip to.remove the sump without trashing it is to undo all the 8mm bolts and then the 2 x 10mm nuts..... When there off undo the 2 studs that the 10mm nuts fit to...... They have a small lip on them on the inner edge of the sump....As you draw the studs down they will pop the sump off with out much effort and leave just a small impression on the face of the sump. This you simply turn upside down and tap flat with the use of a 3/8th drive socket extension bar and then re seal with a decent silicone sealer.
Other points we clean / replace are like mentioned the small oil way gauze filter in the vac pump and also the oil feed pipe to the turbo..... A little awkward to do but remove the turbo if you can and replace the pipe for a new one...... They have a small gauze filter in them too which blocks. Some people have actually removed the gauze filters completely but I think a little bit of maintenance on them from time to time is better than large chunks of carbon floating around the system.
I will add it too have a focus 1.6TDCI and love it to bits...... I change the oil around every 3 months bit I get it free so it's no real issue for me...... But good quality oil and decent filters changed at regular intervals will definitely help keep your car going.