Property split up

My client's wife owns a stake in a property that she previously resided in with her ex-partner (they were not married) some time ago. She has informally agreed that her ex-partner will purchase her share of that jointly owned property for £42 000.
To establish any capital gain do I base the disposal proceeds for capital gains tax on the £42 000 that she has received or £70 000 being 50% of the market value (£140 000)? The amount actually received will be net of her share of the mortgage outstanding (£21 000 being 50% of £42 000) at the date of the agreement so I assume this is added back to arrive at the total proceeds? There is also an endowment linked to the mortgage currently worth approximately £20 000; she will give up her share in this although I understand that no payment...