Unit Trust Capital Gains calculation

I have contributed the same amount monthly to a Unit Trust for over 20 years. I am now requesting to withdraw a partial amount of its current value. How to I calculate the capital gains on the amount I withdraw ? I will also want to do this yearly even though I still pay the monthly premium. The scheme provider can not offer tax advice, what information should they provide me to complete my tax return ?

Hi,
Units in a trust are treated as if they are shares in an ordinary company.  They are declared in a self assessment tax return on a paper SA108, under listed shares and securities or in the online tax return under capital gains in the listed shares and securities option.  
They will be able to provide you with the figure to include on the tax return, so that any gain is taxed accordingly.
Thank you.

Thank you for your reply. I am still having some problems communicating what I need to the company dealing with the Unit Trusts. They just sent me the details of the partial sale of some my units. It includes the total sale price and price per unit. To work out the capital gain surely I would need to know what the cost basis was (which units bought at what price and any fees) . I can’t get them to understand this, or perhaps I am confused. What precisely should I be asking them them to provide me ? Is there a specific term ? They say I will be sent a tax voucher in October, that seems a little vague and a little late for completing self assessment.

Hi Andrew Jackson,
You would appear to be seeking the value of the unit trusts at the date you acquired them and whether there were any cost incurred in acquiring the unit trusts.
There is no specific phrase, but you may wish to use 'acquisition value'.
Thank you.