Right, since rebuilding the top end I pay far closer attention to my water levels. I've noticed that the level in the expansion tank rises when the engine is hot and falls when the engine is cool. The difference is about half an inch. Is this normal because the water is no longer circulating? Everything since the rebuild has gone swimmingly and previous to that I never paid enough attention to know if this is normal?

its ok

That sounds fine to me like, mine does the same, sounds like simple expansion and contraction,as long as the level goes back to its normal level when cold its fine, that's what the expansion bottle is for

Well that does make a certain amount of sense! Just wanted to check. Blowing your head makes you paranoid!

Perfectly normal mate, I marked mine with a hot and cold level after around 2 weeks to allow settlling down, that way you can check the level whenever you like.