Lost DIY Declaration of Trust – effective date and CGT liability?

Hi, I purchased a property with the help of my father in 2014 or so, I had only contributed a small percentage to the purchase price. About a year later, I gave up control of the property (allowed him to rent it out, and gave him the right to instruct me to sell it). He has been renting it out since then, I receive no income and all repairs and other obligations are on him (I have spent no money on the property since then).

There was no CGT on me as this was my primary residence at the time, which was given away. There was no stamp duty due on him as the transfer amount was below SDLT threshold.

My name is however still on the land registry – I am the legal owner. We had never entered any further restriction on the legal title, so he’s not on the title at all.

We had written up a very short agreement back then, extremely DIY (merely a few short paragraphs), which we still have. We later had a used a template deed of trust document, however, this has been lost, and we cannot locate a copy after trying quite hard. I realise this was stupid.

My father wants to have a proper document in place now, as do I. If we draw up a declaration of trust now, I understand the signatures cannot be back dated. However, can it essentially say that this has been in force since 2014, and we are only writing it down to reconfirm the terms? Would we be able to draw up such a document, and would it be effective? We want it to state the transfer happened in 2014 to avoid CGT arising – even though from our eyes the transfer definitely happened then.

We have evidence that he was the beneficial owner – he has copies of tenancy agreements all having only his name, and he may be able to produce statements showing the income went to him. If the property is sold, he would be entitled to all proceeds.

Many thanks

James