My friend had a not dissimilar issue with a relatives house after a death.
The house was in trust to several members of the family, so they had to agreeand they had huge issues with that.
On top of that, they had a plot of land behind the house in a separate arrangement and ownership.
They don't have a road access to this plot, but the deeds for the land have a clear strip of land so they have access to the land which the family had deliberately put into the deeds when the land had been split into separate plots.
What you need to establish is if this is already one or two plots of land on the land registry. (Easily done - there is a free map thingie where you can see the plots and their numbers). That way you can see if a) there are two plots b) if there is a plot with no obvious access without going on someone else's land.
From there the questions are if its a single plot atm, what the plots are going to be (ie. what are you actually buying) and whether there are access conditions.
If its two plots then if they sell one plot blocking access to the other, unless it is specified explicitly in the conditions of sale they DO NOT have a legal right to use your land without permission and you wouldn't give it. Again this needs to be made clear for all parties to prevent disputes further down the line.
My guess is a) they haven't fully thought it through - particularly with regards to power, water and sanitation which they will have to install / split / disconnect (and pay to do prior to the sale) b) they may not have formally split the plot yet c) family members have a difference of opinion and can't agree
I think you have to be direct with the agent, set this out explicitly and give a deadline. If they don't meet the deadline, walk.
It will cause no end of issues if this isn't resolved from the off or there is at least a concerted acknowledgment of these problems from the word go. My friend has now sold the house after the sale fell through twice but she's still trying to resolve issues with other plots of land. This has been going on for about three years in one way or another because the family can't agree.