Continuing on you finally enter the park and there is a little stop called Lewis Lake (by Lewis Lake), which, using the park map, is just inside the Caldera Boundary. It is standing here that you get a sense of how large Yellowstone caldera is (purely because you can't see the whole thing!), something I have always known but haven't truly appreciated. Of course its also excellent to visit all the other well known spots; Old (not so) Faithful, Norris Geyser Basin, Mammoth Hot Springs and the Yellowstone Lake. In the most active spots the faint Sulphur/Hydrogen Sulphide smell was never far away, particularly at the Sulphur Cauldron, although I think Seltun in Iceland smelt slightly worse.
Next time, and I am sure there will be a next time, it would be good to go "off the beaten track" I am sure there are many wonderful parts of the park, such as I have discovered in the Lake District (UK), that can only be accessed on foot.
Note to self - must try and do future research in Yellowstone!