Congrats Lemony and Lowlevelowl
Baart, MATS part 1 is way way more than you need to delve into at any point during the selection process. At stage 1 you need to focus on the document they send you, any further information from other sources will probably be more confusing than helpful. Even at stage 3 (presuming you get that far) the mere fact that you are aware of the existence of MATS part 1 and know what it is should be more than enough to satisfy the interviewers that you've done your research
Of course, a little light reading to satisfy your curiosity won't do any harm, but you are unlikely to be questioned on it!