Im looking to change my parking lights on my 07 MCS as the halogen yellow lights do not look right with the Xenons.

Firstly am I able to use 501 bulbs for a straight swap as im hoping to use some white LEDS?

Secondly, I used to have an 04 Cooper and they were easy to change on there however it seems harder to get to on the MCS. How do you go about changing them?

Changing the bulbs is really easy. Check the link I've provided.
As for LED's, you can change them, but you might get a bulb error on your display.
I just changed the park lights to OSRAM Coolblues to match the tone of the foglights.

http://www.motoringfile.com/files/2007_MINI_Hardtop.pdf