Hi Andrew, this is an interesting one...
I have sourced COMPLETE rear mirror assemblies for forum members in the past (£20) but i have no experience of fitting the actual lens back into the mirror housing/frame.
The soaking in to the hot water sounds a good bet, stick a small amount of wash up liquid in there too as a lubricant...
What comes out, must go back in! (in theory anyway

)
Good luck, and keep us posted?
