I understood that due to the land being part of their garden and less than 1.5hectars they would not be liable for cgt?
I was waiting for you to say that ... I didn't want to "lead the witness". If this is the garden/grounds of a house which has always been their only or main residence and the total area is 0.5Ha (1.24 acres) or less, then yes the disposal will be free of CGT. But it must still be garden/grounds at the date of disposal. If work has already started then relief will not be due.
Call it what you like, but if your parents "gift" you some land with the expectation of receiving a "gift" of money some time later to effectively pay for the "gift" then it isn't a gift. But there is no income tax on gifts anyway, and the only potential tax is inheritance tax if (a) the value of the estate is high enough and (b) one of the "donors" does not live for 7 years after the "gift". But I still don't think that we are dealing with gifts here.