The question isn't clear.
Izz31 wrote:selling
This suggests (future tense) that it hasn't yet been done, but:
Izz31 wrote:I have a deed of trust for my 20%
This suggests that it has been done, such that you did own 100% and now own only 20%.
No stamp duty land tax is payable because it does not exceed the £125k threshold. However an SDLT return needs to be submitted. If, in the future, you transfer the remaining 20% to them (or anybody connected with them) then this may be a linked transaction that would bring this transaction into charge.