Thanks Sam, very useful and clear. To clarify completely though:
If i sell having let the property for less than 18 months after i move out - no capital gains tax is due.
If i sell after (say 2 years) I would get 2.5 years worth tax free (july 2012 - Jan 2015) and 2 years taxed based on the whole increase split between these.
Or would i get the 18 months you mentioned as well? so just 6 months would be taxable?