There is a possibility of having your mark for an individual assignment adjusted but you need to organise a meeting with your tutor/coordinator and preferrably very soon after you received the assignment back (i.e., not at the end of semester). Even then, there is no guarantee.
In one assignment worth 30% of the final subject mark, I received a far lower-than-expected mark and met with the coordinator to discuss it further. He gave me an ultimatum: take a 5% increase or have another tutor re-mark the assignment and risk receiving an even lower (or potentially higher) mark. I took the 5% increase and that ended up making the difference between a H1 and H2A, haha. As I said, the key is to contact them early and be genuinely concerned, rather than act like you're entitled to a higher mark or you need the marks to gain entry to X postgraduate degree. That won't get you anywhere.
For exams, the chance of having your mark redadjusted is very slim unless the coordinator made a genuine error or there was a mistake in the exam question itself.