I haven't ever done a Yammi, but I've worked on numerous mercs and the sequence would be.
Assuming you wish to replace items no. 12.......
Remove gearbox (lower unit)
remove nuts 22
remove nuts 33
probably replace some of items 10, 11, 13
reverse above sequence
I'd imagine that a dealer would charge in excess of 500 pounds to do this job, as things rarely come apart as easy as they should!
It is better to remain silent and be thought a fool, than to speak and remove all doubt!