If I sell my main residence and move into another property I own which was a letting, does it automatically become my primary residence, hence it would not be liable to capital gains tax (CGT)? 

Arthur Weller replies:  

The second residence you move into will certainly become your primary residence. However, that does not necessarily mean it is not liable for CGT. Suppose you have owned it and rented it out for six years, and now you move into it and live there as your primary residence for four years before selling it. When you sell it, 60% of the capital gain will be subject to CGT, and 40% will be exempt.