There are many rules on this. When did you buy, how long has it been let, did you ever live in it yourself, has the value gone down below what you paid?
You could live in it for 6 months as your main residence, yes, that would work or move abroad for 5 years!!
Yes Philip, isn't something similiar to working out the capital gain on shares - not something I've done for donkey's years but remember there were complicated tables of years and values!!!!!!
I know CGT is a very complex arena, however with buy to lets, you get various different % allowances dependant on many things such as if you have lived in the property before you rent or after you rent and a few others that I cannot remember but do have the notes for future reference.