How to complete CGT form on second home jointly owned then wholly owned

I am assisting my father with completing a CGT return following the sale of his second property. Originally he inherited the property in joint ownership with my mother, and then has been the sole owner since her death. We have calculated his share of the gain (50% then 100%) for the relevant periods, and attempted to complete the online HMRC form. However, we got stuck as the online form only seems to allow for a single period of ownership (i.e. you can't split out separate periods of ownership). Please can you advise how to complete the form for the situation I have described? Can you override the gain figure calculated and provide supporting information? And does the form then automatically calculate CGT due based on the income figure provided? Thanks.

Hi 
Unfortunately, the system only allows the user to enter a single date.
Where a person has acquired a single property in parts or instalments and has more than one acquisition date to enter, we would advise that you:
•    enter the earliest acquisition date and acquisition amount and 
•    provide the remaining detail in an upload
You also have the option to either enter: 
•    a single total amount for the acquisition costs or 
•    add the acquisition amounts as improvement costs, such that the inbuilt calculator generates the correct result or 
if this is not possible, you can enter the first acquisition amount and then choose the option:
•    to override the system generated figure and 
•    enter your own calculation of the CGT due. 
In both instances the supporting documentation should be uploaded.

Thank you