Neighbour Demanding walkway in writing.

Our neighbour has an 18 cent plot adjacent to ours. They have been using our land as an ingress to theirs and that is the only way by which they could access their plot, and so we hadn't denied them using it either since it's a legal binding on us to allow them. Now they plan to sell their property and demands they require a land through our plot for access, we agreed that they could use our land for access. But now they demand us to legally give it to them in writing. What is the minimum width of land required to give them access? Can I decide which path I would give them? Is there a law to provide them way to take their cars too? Or is it just width to walk? Is there a need to provide them access through our land in writing? Or can we just provide them an access keeping our Land to ourselve's. (the land costs/cents are around 1.25 lakhs, we had offered them, 1.5 but they ask 2 lakhs which they say another buyer has offered. Which clearly is a lie. What they plan to do is to get our land in writing and then keep it along with theirs without selling) How should I approach this situation?