Tax Champion wrote:Assuming you are not actually married, your partner gifted you a 50% share in the property for no cost, so made no gain at that time.
Not so.
Assuming you are not actually married, your partner gifted you a 50% share in the property at a value DEEMED to be market value and he crystallised a gain at that point based on that value.
If it was his main residence for the entirety of the period of his ownership then he is granted relief from the tax due.