SDLT on later pchse of garden plot following pchse of house

[Pupp]

My partner's mother is to sell her house to us. The value of house and grounds is £295k but she wishes to help us by holding onto part of the garden (that we could freely use in the meantime) and transfer the house and remaining land for £245k. Is there any sort of mechanism that can be used to secure the remainder for a future pchse without it being treated as a 'linked' transaction thus triggering 3% SDLT liability? If not, how long would need to elapse before the later transfer would not be linked? Many thanks.