Any question answered...

I'm looking at big tyres to fit a 700c rim. I found that Schwalbe do their 'big apple' in 28x2.15. Will that fit, or is that specially for 29ers (or is a 29er just a mountain bike with 700c wheels?)

700 = 29er
29er = fat 700 essentially

Should be the same bead seat diameter. Its the width that will cause you issues. How wide is the rim? Can it handle a fat tyre? Standard 19mm road rim can take a 28 usually. I've squeezed a 42 on with no issues. Dunno what 2.15 equates to but I'm guessing its more than 42.