Ok thanks for the speedy replies and help guys, I've done as suggested by "Trickery" and replaced the lines he suggested within the epub and saved the file back, Loaded it back into Calibre and converted to MOBI using the Kindle output profile, Still getting a large left margin

Opening book view in Calibre and you can clearly see this (looks worse on my Kindle3) Perhaps you can tell me where I'm going wrong with this process

Code change in Sigil as suggested by Trickery (thanks)
Book view in Calibre after conversion to MOBI formate:

Same book on Kindle: