Hi,
Yes you only declare your share of the gain.
We cannot comment on why the figures are less than what you anticipated as not enough information has been given.
Submit your tax return with all your details completed.
If you have any further questions, which are personal to you, do not post on this public forum but contact the Self Assessment Helpline on 0300 200 3310 or use the options available on
Self Assessment: general enquiries
Thank you