... They're a great employer, quite generous with their entitlements and my boss is the best I've had in my 8 years there, but I'm not sure if I want to call him and say hey I'm thinking of not coming back, can you work out for me what would happen.
Hey hey, guess what, same boat over here except I took the 12 month extension

So, my approach is going to be the above - call and say I'm thinking about not coming back. Teeki knowing our employer, if you say to boss person "confidentially, this is what I'm thinking" he should be able to provide you some info. That's his job and honestly he probably won't be surprised by it - I think it's pretty common. When I asked for the extension my LM was like "sure no worries I'll get onto it" - totally unsurprised. And I think if I can resign (still making the decision) then he won't be surprised by that either.