How to report property capital gain on varying ownership

[Red5000]

My husband and I bought a property with our son in around the year 2000. We have always owned it jointly but there was a declaration of trust which apportioned the capital interest according to the amount invested by us and our son who then lived there. Until March 2012 my husband and I each owned 32% of the capital in the property. We then bought our son’s 36% and subsequently each owned 50% of the capital. It was our son’s main residence and he took his money to buy a property in his own name. We then let the property and now wish to sell.
I can see that there are two time periods for which the gain has to be calculated but I don't think this is possible to report on the online system for one disposal. It assumes a constant ownership percentage unless there is a way I didn't spot when I used it recently.
Please can you explain to me the mechanism for reporting the capital gain on this single disposal with two distinct periods where the proportion of interest is different.
Many thanks.