Uwila, I think the reason they give the individual jabs in separate legs (and note down which went where, plus batch nos.) is so that if there is a localised reaction (swelling, lumps etc.), they can track down which vaccine caused it.
We had this with dd2 -- developed a small (non-painful) pea-sized lump in her thigh from one of the jabs (forget which). Knowing which one it was meant they could swap legs for the next set and avoid aggravating it.
With the 5-in-one vaccine, it will be a single unit, so the issue doesn't arise.