As 6pointer said, the FC website is good for what your after if your farmer/landowner doesn’t know (although highly unlikely!!)
The link below will hopefully come up with a UK map.
Forestry Commission Scotland Map Viewer -- Basic View
Click in your general area on in the UK then Zoom into desired area with the Tab on the left
Select “
Map Layers” tab on Right hand side
Then tick the select “
Aerial Photography” box near the bottom.
You can use the standard format OS type map representation but I prefer the aerial photography for this.
Select “
Measure Area” Icon on the (further most left tab on the top tool bar)
Once you have a decent Map ratio, left click your mouse around the boundary edge of your particular ground, zoom in as much as possible to let you see as much details as possible.
It will give you the Area in acres/hectares at the top of the column on the left once you've completed your edges.
A wee tip, click as many times as possible to follow the contours of your ground especially if there are little straight edges like irregular fields, river edges or natural plantations etc
There is also a distance tool which can also be useful at times.
It doesn’t take long to get the hang of and is quite a useful tool & you can clear it easily to start again from the left hand column.
Worth a try anyway on a rainy day
Hope that helps
Rigboot